A, and C are incorrect. These all choices itself comes under risk action plan. As in the described scenario, risk action plan is not turned to be effective, these should not be implemented again.

Residual risk is the risk that remains after security controls have been implemented on a system. Residual risk can be accepted, transferred, avoided, or further mitigated. The most important consideration when deciding whether to accept residual risk is the cost versus benefit of additional mitigating controls. This means comparing the potential impact of the residual risk with the cost and effectiveness of implementing more controls to reduce it. If the cost of additional controls outweighs the benefit of reducing the residual risk, then it may be acceptable to accept the residual risk. However, if the benefit of additional controls exceeds the cost, then it may be advisable to implement more controls to lower the residual risk to an acceptable level. References = Risk and Information Systems Control Study Manual, Chapter 3: Risk Response and Mitigation, Section 3.4: Risk Response Selection, p. 156-157.

The percentage of relevant threats mitigated is the best key control indicator (KCI) to determine the effectiveness of an intrusion prevention system (IPS), because it measures how well the IPS is performing its intended function of preventing unauthorized access or attacks. The percentage of system uptime is not a good KCI, because it does not reflect the quality or accuracy of the IPS. The total number of threats identified is not a good KCI, because it does not indicate how many of those threats were actually prevented by the IPS. The reaction time of the system to threats is not a good KCI, because it does not measure the impact or severity of the threats that were prevented or not prevented by the IPS. References = CRISC: Certified in Risk & Information Systems Control Sample Questions2