The BEST way for management to validate whether risk response activities have been completed is...

Reviewing the risk register change log is the best way for management to validate whether risk response activities have been completed, because it helps to track and monitor the changes and updates that have been made to the risk register, and to verify that the risk response activities have been implemented and closed. A risk register is a document that captures, identifies, assesses and tracks risk as part of the risk management process4. A risk register change log is a record that documents the date, description, and reason for each change or update that is made to the risk register. A risk response activity is an action or task that is performed to implement the chosen risk response strategy for a specific risk, such as avoid, transfer, mitigate, or accept. Reviewing the risk register change log is the best way, as it helps to ensure that the risk register is accurate and current, and that the risk response activities have been completed and reported. Reviewing evidence of risk acceptance, control effectiveness test results, and control design documentation are all possible ways to validate whether risk response activities have been completed, but they are not the best way, as they may not cover all the risk response activities, and they may not reflect the changes or updates in the risk register. References = Risk and Information Systems Control Study Manual, Chapter 3, Section 3.4.1, page 101