Explanation: A zero-day attack, also known as zero-hour attack, is a computer threat that tries to exploit computer application vulnerabilities which are unknown to others, undisclosed to the software vendor, or for which no security fix is available. Zero-day exploits (actual code that can use a security hole to carry out an attack) are used or shared by attackers before the software vendor knows about the vulnerability. User awareness training is the most effective technique to mitigate such attacks. Answer: A is incorrect. A replay attack is a type of attack in which attackers capture packets containing passwords or digital signatures whenever packets pass between two hosts on a network. In an attempt to obtain an authenticated connection, the attackers then resend the captured packet to the system. In this type of attack, the attacker does not know the actual password, but can simply replay the captured packet. Answer: C is incorrect. Man-in-the-middle attacks occur when an attacker successfully inserts an intermediary software or program between two communicating hosts. The intermediary software or program allows attackers to listen to and modify the communication packets passing between the two hosts. The software intercepts the communication packets and then sends the information to the receiving host. The receiving host responds to the software, presuming it to be the legitimate client. Answer: D is incorrect. A Denial-of-Service (DoS) attack is mounted with the objective of causing a negative impact on the performance of a computer or network. It is also known as network saturation attack or bandwidth consumption attack. Attackers perform DoS attacks by sending a large number of protocol packets to a network.