A Nessus scan reports a CVSS 9.

CEH v13 states that vulnerabilities with a CVSS score ≥ 9.0 are critical and require immediate containment. When remote code execution is possible, the system may already be compromised.

The recommended response is to isolate the affected system, preventing lateral movement, then perform a forensic audit before applying patches. Immediate patching without isolation may alert attackers or destroy evidence.

Thus, option D aligns with CEH incident response best practices.