Alice, a system administrator, notices that some sensitive files have been accessed by unauthorized users. She wants to ensure that, in the future, only specific users can view and modify these files. What security measure should Alice implement?

Alice should implement Access Control Lists (ACLs) to restrict file access. ACLs define which users or system processes are granted access to objects, as well as what operations are allowed on given objects. In this case, Alice can specify which specific users have access to sensitive files and what type of access they have.