Isolating impacted systems (A) is the most critical consideration for effective incident containment because it directly limits spread, escalation, and additional damage. CISM incident management guidance prioritizes technical containment actions that preserve evidence and protect unaffected systems. Reputational (B) and financial (C) concerns are important but are outcomes of how well containment is executed. Assigning accountability (D) supports governance but does not directly contain an incident. Rapid isolation is essential to reduce overall impact and enable effective eradication and recovery. References: ISACA CISM Review Manual (Incident management-containment strategies); CISM Exam Content Outline (Domain 4).