The average time to identify critical information security incidents is the best indicator of effective information security incident management, as it measures how quickly the organization can detect and respond to the most severe threats to its information assets. The faster the organization can identify critical incidents, the lower the potential impact and damage they can cause. The average time to identify critical incidents also reflects the efficiency and accuracy of the monitoring and reporting mechanisms, as well as the awareness and training of the staff and stakeholders. The other options are not the best indicators of effective information security incident management, as they do not directly measure the performance or quality of the incident response process. The monthly trend of information security-related incidents may show the frequency or severity of the incidents, but not how well they are managed. The frequency of information security incident response plan testing may show the preparedness or maturity of the organization, but not the actual effectiveness of the plan. The percentage of high-risk security incidents may show the risk exposure or appetite of the organization, but not the risk mitigationor treatment. References = 7 Types of Information Security Incidents and How to Respond; Annex A.16: Information Security Incident Management - ISMS.online; CISM Isaca Exam Questions and Answers - CertLibrary.com