Explanation (aligned to IH&R lifecycle):

This question is about triage/validation---determining whether what you see is truly an incident and establishing priority. The most appropriate first move is to use endpoint telemetry and behavioral analytics (A) to validate maliciousness (e.g., suspicious parent/child process chains, token manipulation, credential dumping patterns, anomalous privilege escalation, and data transfer behaviors). This supports fast, evidence-based classification and reduces unnecessary disruption. Option (C) is containment and may be required after validation or for clearly high-confidence cases, but immediately disconnecting multiple endpoints can destroy volatile evidence, break business operations, and reduce your ability to trace lateral movement patterns across hosts. Option (B) is a broad preventive change that can create outage risk and is not a validation method. Option (D) can be helpful, but it is slower and not the primary ''detect and validate'' action for an internal team facing active anomalies.

A disciplined approach is: validate via behavioral tooling + logs, scope affected endpoints, determine severity, then execute containment proportional to confirmed risk. That sequencing mirrors standard incident handling flow (identify validate/triage contain eradicate recover lessons learned). When time matters, the highest-value action is the one that converts ambiguous signals into confident incident classification quickly---behavioral validation does that best.