The best way to mitigate an identified risk scenario is to execute a risk response plan. A risk response plan is a document that describes the actions and resources that are needed to address the risk scenario. A risk response plan can include one or more of the following strategies: avoid, transfer, mitigate, accept, or exploit. By executing a risk response plan, the organization can reduce the likelihood and/or impact of the risk scenario, or take advantage of the opportunities that the risk scenario may present. The other options are not as effective as executing a risk response plan, as they are related to the awareness, assessment, or monitoring of the risk scenario, not the actual treatment of the risk scenario. References = Risk and Information Systems Control Study Manual, Chapter 3: IT Risk Response, Section 3.2: IT Risk Response Options, page 133.

The best metric to verify adherence to the policy that requires critical security patches to be deployed in production within three weeks of patch availability is the maximum time gap between patch availability and deployment, as it measures the longest duration that the organization takes to apply the patches, and ensures that it does not exceed the policy limit. The other options are not the best metrics, as they may not reflect the actual or optimal compliance with the policy, or may not be relevant or measurable for the policy, respectively. References = CRISC Review Manual, 7th Edition, page 110.

A post-implementation RCSA is a process of verifying whether the risk treatment plan has been executed as intended and whether the residual risk is within the acceptable level. It involves testing the effectiveness of the controls that have been implemented to mitigate the risk and identifying any gaps or issues that need to be addressed. A BIA, the original risk response plan, and the training program and user awareness documentation are not sufficient to validate the effectiveness of the risk treatment plan, as they do not measure the actual performance of the controls or the residual risk.
References:
*ISACA, Risk IT Framework, 2nd Edition, 2019, p. 761
*ISACA, Risk and Information Systems Control Review Manual, 7th Edition, 2020, p. 2112