The questions specifically said: "within a different function" which eliminate
Job Rotation as a choice.
Management monitoring of audit logs is a detective control and it would not prevent
collusion.
Changing passwords regularly would not prevent such attack.
This question validates if you understand the concept of separation of duties and least privilege. By having operators that have only the minimum access level they need and only what they need to do their duties within a company, the operations personnel would be force to use collusion to defeat those security mechanism. Source: TIPTON, Hal, (ISC)2, Introduction to the CISSP Exam presentation.

Serial Line IP (SLIP) was developed in 1984 to support TCP/IP networking
over low-speed serial interfaces.
Source: KRUTZ, Ronald L. & VINES, Russel D., The CISSP Prep Guide: Mastering the
Ten Domains of Computer Security, John Wiley & Sons, 2001, Chapter 3:
Telecommunications and Network Security (page 114).

Controlling access to information systems and associated networks is necessary for the preservation of their confidentiality, integrity and availability.
Source: KRUTZ, Ronald L. & VINES, Russel D., The CISSP Prep Guide: Mastering the Ten Domains of Computer Security, 2001, John Wiley & Sons, Page 31.

Explanation/Reference:
SMTP (Simple Mail Transfer Protocol) works as a message transfer agent.
Source: HARRIS, Shon, All-In-One CISSP Certification Exam Guide, McGraw-Hill/Osborne, 2001, Page
821.

To the untrusted host, all traffic seems to originate from the proxy server and addresses on the trusted network are not revealed.
"User base" is incorrect. The proxy hides the origin of the request from the untrusted host.
"Operating system design" is incorrect. The proxy hides the origin of the request from the untrusted host.
"Net BIOS' design" is incorrect. The proxy hides the origin of the request from the untrusted host.
References:
CBK, p. 467 AIO3, pp. 486 - 490