C. follow the incident response plan.

Following the incident response plan is crucial because it provides a structured approach for assessing and responding to security events. The plan outlines the steps to take when an incident is suspected, including initial assessment, escalation procedures, and notification protocols. By following the incident response plan, the security manager can ensure that the suspected event is properly evaluated and classified based on established criteria and procedures. This step helps to maintain consistency and effectiveness in the organization's response to security incidents. Once the incident response plan has been followed and the event is confirmed as a security incident, further actions such as notifying the business process owner and conducting forensic analysis can be initiated as necessary, in accordance with the plan.