Explanation
An advanced persistent threat (APT) is a stealthy and sophisticated attack that aims to compromise and maintain access to a target network or system over a long period of time, often for espionage or sabotage purposes. APTs are difficult to detect by conventional security tools, such as antivirus or firewalls, that rely on signatures or rules to identify threats. Therefore, the best way to monitor for APTs is to search for anomalies in the environment, such as unusual network traffic, user behavior, file activity, or system configuration changes, that may indicate a compromise or an ongoing attack. References: https://www.isaca.org/credentialing/cism
https://www.nist.gov/publications/information-security-handbook-guide-managers

Section: INFORMATION SECURITY PROGRAM MANAGEMENT

Explanation
A layered defense strategy would only prevent those activities that are outside of the user's privileges. A signed acceptable use policy is often an effective deterrent against malicious activities because of the potential for termination of employment and/or legal actions being taken against the individual. System audit log monitoring is after the fact and may not be effective. High-availability systems have high costs and are not always feasible for all devices and components or systems.