User recertification is the most effective control to ensure user access is maintained on a least-privilege basis, as it involves a periodic review and validation of user access rights and privileges by the appropriate authority. User recertification helps to identify and remove any unnecessary, excessive, or obsolete access rights and privileges that may pose a security risk or violate the principle of least privilege. User recertification also helps to ensure that user access rights and privileges are aligned with the current business needs, roles, and responsibilities of the users.

The other options are not the most effective controls to ensure user access is maintained on a least-privilege basis. User authorization is the process of granting or denying access rights and privileges to users based on their identity, role, and credentials, but it does not verify or update the existing access rights and privileges of the users. Change log review is the process of examining and analyzing the records of changes made to the system, configuration, or data, but it does not directly address the user access rights and privileges. Access log monitoring is the process of tracking and auditing the user activities and actions on the system or network, but it does not validate or modify the user access rights and privileges. References = What Is the Principle of Least Privilege and Why is it Important?, Principle of Least Privilege: Definition, Methods & Examples, IT Risk Resources | ISACA