'Mean time to remediate (MTTR)' is the correct answer, as this KPI measures the average time taken to resolve a security breach once it has been detected. A decreasing trend in MTTR over time would indicate that the organization is becoming more efficient at remediating issues, thereby reducing the impact of security breaches. 'Mean time to detect (MTTD)' focuses on detection rather than remediation, and while 'Change window duration' and 'Incident count' can provide supplemental information, they do not directly measure the efficiency of remediation processes.