The correct answer is C because the primary purpose of an incident response team is to respond to security incidents and reduce their impact through effective mitigation. Incident response teams investigate incidents, classify severity, contain threats, coordinate response actions, support eradication, assist recovery, and help prevent recurrence through lessons learned. A business impact analysis is generally part of business continuity planning, not the primary responsibility of the incident response team. A risk assessment for zero-day vulnerabilities is part of risk management and vulnerability management, not the core incident response function. Acting as a single point of contact for critical incidents may be one responsibility of the team or incident coordinator, but it is not the primary function. CISM incident management emphasizes timely detection, response, containment, communication, recovery, and post-incident improvement. The team exists mainly to reduce business harm from incidents. Therefore, providing effective incident mitigation is the best answer.
Reference: CISM Information Security Incident Management; incident response team responsibilities, mitigation, containment, and impact reduction principles.