Residual risk is the risk that remains after the risk mitigation plans have been implemented. Residual risk reflects the effectiveness of the risk response in reducing the likelihood or impact of the risk. The best evidence that risk mitigation plans have been implemented effectively is the change in the level of residual risk. A change in the level of residual risk can be measured by comparing the risk level before and after the risk mitigation plans have been executed. A change in the level of residual risk can also be evaluated by comparing the actual residual risk with the target or acceptable residual risk. A change in the level of residual risk can demonstrate how well the risk mitigation plans have achieved the risk objectives and met the risk criteria. A change in the level of residual risk can also provide feedback and lessons learned for future risk management activities. References = Residual Risk: Definition, Formula & Management, Residual Risk:
What It Is and How to Manage It, Residual Risk: How to Calculate and Manage It.

The control owner should be responsible for evaluating the residual risk after a compensating control has been implemented. A compensating control is a control that provides an alternative or additional measure of protection when the primary or preferred control is not feasible or effective. A residual risk is the risk that remains after the risk response or mitigation has been applied. The control owner is the person who has the authority and responsibility for designing, implementing, and monitoring the controls that enforce the policy.
The control owner can assess the impact and effectiveness of the compensating control on the residual risk, and report the results and recommendations to the risk owner or the risk practitioner. The other options are not as responsible as the control owner, as they are related to the compliance, ownership, or management of the risk, not the evaluation of the control. References = Risk and Information Systems Control Study Manual, Chapter 4: Risk and Control Monitoring and Reporting, Section 4.4: Key Control Indicators, page 211.

Residual risk is the risk that remains after the risk response has been implemented. Risk tolerance is the acceptable level of variation from the desired outcome or objective. If the residual risk is within the risk tolerance, it means that the risk response has been effective in reducing the risk to an acceptable level.
References
*ISACA CRISC Review Manual, 7th Edition, Domain 3: Risk Response, Section 3.1.1: Residual Risk
*Residual Risk: Definition, Formula & Management - Video & Lesson Transcript | Study.com
*Risk Tolerance - ISACA

A risk register is a document that records and tracks the information and status of the identified risks and their responses. It includes the risk description, category, source, cause, impact, probability, priority, response, owner, action plan, status, etc.
A risk register update is a change or modification to the information or status of the risks and their responses in the risk register. It may be triggered by the occurrence or resolution of a risk event, the identification or evaluation of a new or emerging risk, the implementation or completion of a risk response, the monitoring or review of the risk performance, etc.
The most important risk register update for senior management to review is avoiding a risk that was previously accepted, which means that the organization has decided to eliminate or withdraw from the risk exposure or activity that may cause the risk, instead of tolerating or retaining the risk as before. This may indicate a significant change in the organization's risk appetite, strategy, objectives, or environment, and it may have a major impact on the organization's performance and value.
The other options are not the most important risk register updates for senior management to review, because they do not indicate a significant change or impact on the organization's risk profile or performance.
Extending the date of a future action plan by two months means that the organization has postponed the implementation or completion of the planned actions or measures to address the risk, due to some reasons or constraints. This may indicate a delay or deviation from the expected or desired risk outcome, but it may not have a major impact on the organization's performance and value, unless the risk is very urgent or critical.
Retiring a risk scenario no longer used means that the organization has removed or discarded the risk scenario that is no longer relevant or applicable to the organization's objectives or operations, due to some changes or developments. This may indicate a reduction or improvement in the organization's risk exposure or level, but it may not have a major impact on the organization's performance and value, unless the risk scenario was very significant or influential.
Changing a risk owner means that the organization has assigned or transferred the responsibility and accountability for the risk and its response to a different person or role, due to some reasons or circumstances.
This may indicate a change or improvement in the organization's risk governance or culture, but it may not have a major impact on the organization's performance and value, unless the risk owner was very ineffective or inappropriate. References = ISACA, CRISC Review Manual, 7th Edition, 2022, pp. 19-20, 23-24, 27-28, 31-32, 40-41, 47-48, 54-55, 58-
59, 62-63
ISACA, CRISC Review Questions, Answers & Explanations Database, 2022, QID 160 CRISC Practice Quiz and Exam Prep