This scenario describes Living-off-the-Land (LotL) malware techniques, where attackers modify or abuse legitimate system binaries and services to evade detection. CEH v13 identifies this as a highly stealthy persistence mechanism commonly used in advanced persistent threats (APTs).

The most effective countermeasure is file integrity monitoring (FIM), specifically by tracking cryptographic hashes of critical system executables. CEH v13 emphasizes that monitoring file hashes enables early detection of unauthorized modifications to binaries such as PowerShell, cmd.exe, or Windows services.

Backups (Option A) aid recovery but do not prevent or detect compromise. Antivirus updates (Option C) often fail against modified legitimate tools. Firewall hardening (Option D) reduces attack surface but does not detect tampering of trusted binaries.

CEH v13 explicitly recommends hash-based integrity verification as a core defense against stealthy persistence mechanisms. Therefore, option B is correct.