In a scenario where a production batch job failed after modifications post-user acceptance testing, which control is most effective to prevent future risks?

The most effective control to prevent future risks in the scenario provided is to ensure that developers do not have access to code after testing. This measure directly addresses the potential for developers to introduce errors or unauthorized changes after the code has been finalized and accepted through user testing. It establishes a clear separation of duties, which is essential in managing risks related to production environments.

By restricting access, you minimize the risk of untested changes being deployed, which can lead to production failures. This control helps to maintain the integrity of the system and ensures that only those who are authorized and have gone through the proper change management processes can make updates or modifications after a release.

In contrast, improving regression test cases, while beneficial, may not be sufficient alone if the development team can still make changes post-testing that bypass these tests. Activating audit trails for a limited period is useful for tracking changes but does not prevent unauthorized changes from occurring. Conducting an application user access review is a good practice for overall security but does not specifically mitigate the risk of post-testing code modifications that led to the job's failure.