The Answer: Session layer, which establishes, maintains and manages sessions and synchronization of data flow. Session layer protocols control application-to-application communications, which is what an RPC call is.
The following answers are incorrect:
Transport layer: The Transport layer handles computer-to computer communications, rather than application-to-application communications like RPC.
Data link Layer: The Data Link layer protocols can be divided into either Logical Link Control (LLC) or Media Access Control (MAC) sublayers. Protocols like SLIP, PPP, RARP and L2TP are at this layer. An application-to-application protocol like RPC would not be addressed at this layer.
Network layer: The Network Layer is mostly concerned with routing and addressing of information, not application-to-application communication calls such as an RPC call.
The following reference(s) were/was used to create this question: The Remote Procedure Call (RPC) protocol is implemented at the Session layer, which establishes, maintains and manages sessions as well as synchronization of the data flow. Source: Jason Robinett’s CISSP Cram Sheet: domain2.
Source: Shon Harris AIO v3 pg. 423