The reference monitor refers to abstract machine that mediates all access to objects by subjects.
This question is asking for the concept that governs access by subjects to objects, thus the reference monitor is the best answer. While the security kernel is similar in nature, it is what actually enforces the concepts outlined in the reference monitor.
In operating systems architecture a reference monitor concept defines a set of design requirements on a reference validation mechanism, which enforces an access control policy over subjects’ (e.g., processes and users) ability to perform operations (e.g., read and write) on objects (e.g., files and sockets) on a system. The properties of a reference monitor are:
The reference validation mechanism must always be invoked (complete mediation). Without this property, it is possible for an attacker to bypass the mechanism and violate the security policy.
The reference validation mechanism must be tamperproof (tamperproof). Without this property, an attacker can undermine the mechanism itself so that the security policy is not correctly enforced.
The reference validation mechanism must be small enough to be subject to analysis and tests, the completeness of which can be assured (verifiable). Without this property, the mechanism might be flawed in such a way that the policy is not enforced.
For example, Windows 3.x and 9x operating systems were not built with a reference monitor, whereas the Windows NT line, which also includes Windows 2000 and Windows XP, was designed to contain a reference monitor, although it is not clear that its properties (tamperproof, etc.) have ever been independently verified, or what level of computer security it was intended to provide.
The claim is that a reference validation mechanism that satisfies the reference monitor concept will correctly enforce a system’s access control policy, as it must be invoked to mediate all security-sensitive operations, must not be tampered, and has undergone complete analysis and testing to verify correctness. The abstract model of a reference monitor has been widely applied to any type of system that needs to enforce access control, and is considered to express the necessary and sufficient properties for any system making this security claim.
According to Ross Anderson, the reference monitor concept was introduced by James Anderson in an influential 1972 paper.
Systems evaluated at B3 and above by the Trusted Computer System Evaluation Criteria (TCSEC) must enforce the reference monitor concept.
The reference monitor, as defined in AIO V5 (Harris) is: “an access control concept that refers to an abstract machine that mediates all access to objects by subjects.”
The security kernel, as defined in AIO V5 (Harris) is: “the hardware, firmware, and software elements of a trusted computing based (TCB) that implement the reference monitor concept. The kernel must mediate all access between subjects and objects, be protected from modification, and be verifiable as correct.”
The trusted computing based (TCB), as defined in AIO V5 (Harris) is: “all of the protection mechanisms within a computer system (software, hardware, and firmware) that are responsible for enforcing a security policy.”
The security domain, “builds upon the definition of domain (a set of resources available to a subject) by adding the fact that resources withing this logical structure (domain) are working under the same security policy and managed by the same group.”
The following answers are incorrect:
“The security kernel” is incorrect. One of the places a reference monitor could be implemented is in the security kernel but this is not the best answer.
“The trusted computing base” is incorrect. The reference monitor is an important concept in the TCB but this is not the best answer.
“The security domain is incorrect.” The reference monitor is an important concept in the security domain but this is not the best answer.
Reference(s) used for this question: Official ISC2 Guide to the CBK, page 324
AIO Version 3, pp. 272 -274 AIOv4 Security Architecture and Design (pages 327 -328) AIOv5 Security Architecture and Design (pages 330 -331)
Wikipedia article at https://en.wikipedia.org/wiki/Reference_monitor

A system with multiple processors is called a multiprocessing system.
Multitasking is incorrect. Multitasking involves sharing the processor amoung all ready processes. Though it appears to the user that multiple processes are executing at the same time, only one process is running at any point in time.
Multithreading is incorrect. The developer can structure a program as a collection of independent threads to achieve better concurrency. For example, one thread of a program might be performing a calculation while another is waiting for additional input from the user.
“Parallel running” is incorrect. This is not a real term and is just a distraction.
References:
CBK, pp. 315-316 AIO3, pp. 234 -239

In computer science, hierarchical protection domains, often called protection rings, are mechanisms to protect data and functionality from faults (fault tolerance) and malicious behaviour (computer security). This approach is diametrically opposite to that of capability-based security.
Computer operating systems provide different levels of access to resources. A protection ring is one of two or more hierarchical levels or layers of privilege within the architecture of a computer system. This is generally hardware-enforced by some CPU architectures that provide different CPU modes at the hardware or microcode level.
Rings are arranged in a hierarchy from most privileged (most trusted, usually numbered zero) to least privileged (least trusted, usually with the highest ring number). On most operating systems, Ring 0 is the level with the most privileges and interacts most directly with the physical hardware such as the CPU and memory.
Special gates between rings are provided to allow an outer ring to access an inner ring’s resources in a predefined manner, as opposed to allowing arbitrary usage. Correctly gating access between rings can improve security by preventing programs from one ring or privilege level from misusing resources intended for programs in another. For example, spyware running as a user program in Ring 3 should be prevented from turning on a web camera without informing the user, since hardware access should be a Ring 1 function reserved for device drivers. Programs such as web browsers running in higher numbered rings must request access to the network, a resource restricted to a lower numbered ring.
“They provide strict boundaries and definitions on what the processes that work within each ring can access” is incorrect. This is in fact one of the characteristics of a ring protection system.
“Programs operating in inner rings are usually referred to as existing in a privileged mode” is incorrect. This is in fact one of the characteristics of a ring protection system.
“They support the CIA triad requirements of multitasking operating systems” is incorrect. This is in fact one of the characteristics of a ring protection system.
Reference(s) used for this question:
CBK, pp. 310-311 AIO3, pp. 253-256 AIOv4 Security Architecture and Design (pages 308 -310) AIOv5 Security Architecture and Design (pages 309 -312)

Virtual storage a service provided by the operating system where it uses a combination of RAM and disk storage to simulate a much larger address space than is actually present. Infrequently used portions of memory are paged out by being written to secondary storage and paged back in when required by a running program.
Most OS’s have the ability to simulate having more main memory than is physically available in the system. This is done by storing part of the data on secondary storage, such as a disk. This can be considered a virtual page. If the data requested by the system is not currently in main memory, a page fault is taken. This condition triggers the OS handler. If the virtual address is a valid one, the OS will locate the physical page, put the right information in that page, update the translation table, and then try the request again. Some other page might be swapped out to make room. Each process may have its own separate virtual address space along with its own mappings and protections.
The following are incorrect answers: Primary storage is incorrect. Primary storage refers to the combination of RAM, cache and the processor registers. Primary Storage The data waits for processing by the processors, it sits in a staging area called primary storage. Whether implemented as memory, cache, or registers (part of the CPU), and regardless of its location, primary storage stores data that has a high probability of being requested by the CPU, so it is usually faster than long-term, secondary storage. The location where data is stored is denoted by its physical memory address. This memory register identifier remains constant and is independent of the value stored there. Some examples of primary storage devices include random-access memory (RAM), synchronous dynamic random-access memory (SDRAM), and read-only memory (ROM). RAM is volatile, that is, when the system shuts down, it flushes the data in RAM although recent research has shown that data may still be retrievable. Contrast this
Secondary storage is incorrect. Secondary storage holds data not currently being used by the CPU and is used when data must be stored for an extended period of time using high-capacity, nonvolatile storage. Secondary storage includes disk, floppies, CD’s, tape, etc. While secondary storage includes basically anything different from primary storage, virtual memory’s use of secondary storage is usually confined to high-speed disk storage.
Real storage is incorrect. Real storage is another word for primary storage and distinguishes physical memory from virtual memory.
Reference(s) used for this question:
Hernandez CISSP, Steven (2012-12-21). Official (ISC)2 Guide to the CISSP CBK, Third Edition ((ISC)2 Press) (Kindle Locations 17164-17171). Auerbach Publications. Kindle Edition.
Hernandez CISSP, Steven (2012-12-21). Official (ISC)2 Guide to the CISSP CBK, Third Edition ((ISC)2 Press) (Kindle Locations 17196-17201). Auerbach Publications. Kindle Edition.
Hernandez CISSP, Steven (2012-12-21). Official (ISC)2 Guide to the CISSP CBK, Third Edition ((ISC)2 Press) (Kindle Locations 17186-17187). Auerbach Publications. Kindle Edition.

Procedures are step-by-step instructions in support of of the policies, standards, guidelines and baselines. The procedure indicates how the policy will be implemented and who does what to accomplish the tasks.”
Standards is incorrect. Standards are a “Mandatory statement of minimum requirements that support some part of a policy, the standards in this case is your own company standards and not standards such as the ISO standards”
Guidelines is incorrect. “Guidelines are discretionary or optional controls used to enable individuals to make judgments with respect to security actions.”
Baselines is incorrect. Baselines “are a minimum acceptable level of security. This minimum is implemented using specific rules necessary to implement the security controls in support of the policy and standards.” For example, requiring a password of at leat 8 character would be an example. Requiring all users to have a minimun of an antivirus, a personal firewall, and an anti spyware tool could be another example.
References:
CBK, pp. 12 -16. Note especially the discussion of the “hammer policy” on pp. 16-17 for the differences between policy, standard, guideline and procedure. AIO3, pp. 88-93.

This responsibility would be delegated to a data custodian rather than being performed directly by the information owner.
“Determine what level of classification the information requires” is incorrect. This is one of the major responsibilities of an information owner.
“Periodically review the classification assignments against business needs” is incorrect. This is one of the major responsibilities of an information owner.
“Delegates responsibility of maintenance of the data protection mechanisms to the data custodian” is incorrect. This is a responsibility of the information owner.
References: CBK p. 105. AIO3, p. 53-54, 960

Information classification should be based on the value of the information to the organization and its sensitivity (reflection of how much damage would accrue due to disclosure).
Age is incorrect. While age might be a consideration in some cases, the guiding principles should be value and sensitivity.
Useful life. While useful lifetime is relevant to how long data protections should be applied, the classification is based on information value and sensitivity.
Personal association is incorrect. Information classification decisions should be based on value of the information and its sensitiviry.
References CBK, pp. 101 -102.

Typically there are three to four levels of information classification used by most organizations: Confidential: Information that, if released or disclosed outside of the organization, would create severe problems for the organization. For example, information that provides a competitive advantage is important to the technical or financial success (like trade secrets, intellectual property, or research designs), or protects the privacy of individuals would be considered confidential. Information may include payroll information, health records, credit information, formulas, technical designs, restricted regulatory information, senior management internal correspondence, or business strategies or plans. These may also be called top secret, privileged, personal, sensitive, or highly confidential. In other words this information is ok within a defined group in the company such as marketing or sales, but is not suited for release to anyone else in the company without permission.
The following answers are incorrect:
Public: Information that may be disclosed to the general public without concern for harming the company, employees, or business partners. No special protections are required, and information in this category is sometimes referred to as unclassified. For example, information that is posted to a company’s public Internet site, publicly released announcements, marketing materials, cafeteria menus, and any internal documents that would not present harm to the company if they were disclosed would be classified as public. While there is little concern for confidentiality, integrity and availability should be considered.
Internal Use Only: Information that could be disclosed within the company, but could harm the company if disclosed externally. Information such as customer lists, vendor pricing, organizational policies, standards and procedures, and internal organization announcements would need baseline security protections, but do not rise to the level of protection as confidential information. In other words, the information may be used freely within the company but any unapproved use outside the company can pose a chance of harm.
Restricted: Information that requires the utmost protection or, if discovered by unauthorized personnel, would cause irreparable harm to the organization would have the highest level of classification. There may be very few pieces of information like this within an organization, but data classified at this level requires all the access control and protection mechanisms available to the organization. Even when information classified at this level exists, there will be few copies of it
Reference(s) Used for this question: Hernandez CISSP, Steven (2012-12-21). Official (ISC)2 Guide to the CISSP CBK, Third Edition ((ISC)2 Press) (Kindle Locations 952-976). Auerbach Publications. Kindle Edition.

Baselines provide the minimum level of security necessary throughout the organization.
Standards specify how hardware and software products should be used throughout the organization.
Procedures are detailed step-by-step instruction on how to achieve certain tasks.
Guidelines are recommendation actions and operational guides to personnel when a specific standard does not apply. Source: HARRIS, Shon, All-In-One CISSP Certification Exam Guide, McGraw-Hill/Osborne, 2002, chapter 3: Security Management Practices (page 94).

The following answers are incorrect because:
Accountability, confidentiality and integrity is not the correct answer as Accountability is not one of the fundamental principle of security.
Integrity, availability and accountability is not the correct answer as Accountability is not one of the fundamental principle of security.
Availability, accountability and confidentiality is not the correct answer as Accountability is not one of the fundamental objective of security.
References : Shon Harris AIO v3 , Chapter -3: Security Management Practices , Pages : 49-52