According to CEH v13 Security Operations and Incident Response, backdoors are stealth mechanisms that allow attackers persistent access. Indicators such as unexplained outbound traffic, unauthorized file modifications, and irregular reboots strongly suggest post-compromise persistence mechanisms.

CEH v13 recommends a behavioral and host-based detection approach for backdoor identification. Continuous monitoring of system and file activity helps detect unauthorized binaries, registry changes, and scheduled tasks. Anomaly detection identifies deviations from normal system behavior, which is critical for uncovering hidden backdoors that evade signature-based detection.

Additionally, advanced anti-malware tools with heuristic and memory analysis capabilities are essential to identify sophisticated backdoors that traditional antivirus may miss. These tools can detect rootkits, fileless persistence, and covert communication channels.

The other options are preventative but not investigative. Immediate reboots may destroy volatile evidence, while password policies and ACLs do not detect existing compromises. Therefore, option B provides the most effective and CEH-aligned response.