An organization's vulnerability management team takes an average of 45 days to remediate critical vulnerabilities. Which metric BEST measures the effectiveness of the remediation program?

B is correct because MTTR for critical vulnerabilities directly measures remediation program effectiveness by showing how long the organization remains exposed to critical risks. Comparing against SLAs shows compliance; comparing against benchmarks shows relative program maturity. A is wrong because total vulnerabilities discovered measures discovery capability, not remediation effectiveness. C is wrong because scanner count is an input metric, not an outcome metric. D is wrong because scan completion measures program execution, not remediation outcomes.