The answer 'Mean time to remediate (MTTR)' is correct because it directly reflects the efficiency and effectiveness of the vulnerability management process by measuring the average time taken from when a vulnerability is identified until it is remediated. Monitoring the MTTR over time can help identify improvements or degradations in the vulnerability management processes. 'Alert volume' does not necessarily indicate effectiveness, as it could reflect a high number of false positives or an increase in scanning activities. 'Mean time to detect (MTTD)' measures how quickly vulnerabilities are identified, not how efficiently they are managed or resolved. 'Critical vulnerabilities and zero-days' is a metric that indicates the presence of high-risk vulnerabilities but does not reflect the improvements in managing vulnerabilities over time.