What is the purpose of the Cleanup Rule in a security policy?

The correct answer is D. A Cleanup Rule is placed at the bottom of a rulebase or layer to handle traffic that did not match any earlier explicit rule. In a secure Access Control Policy, its usual purpose is to drop or reject all unmatched traffic and, as a best practice, log that traffic for investigation. Option A is the opposite of a secure cleanup rule because accepting unmatched traffic defeats positive-control policy design. Option B is incomplete: cleanup rules can log unmatched traffic, but logging is not the primary enforcement action. Option C is wrong because “known malicious traffic” is handled primarily by Threat Prevention protections; the cleanup rule deals with unmatched traffic, whether malicious or simply unauthorized. The cleanup rule is important because it makes the default-deny posture visible and auditable rather than relying silently on an implicit cleanup rule. Reference topics: Cleanup Rule, Explicit Cleanup Rule, Access Control Policy, positive-control firewall model.