Final answer:
Hashing, Read-only attributes, and ACLs are controls that can be added to verify on a periodic basis that the files were not modified.
Step-by-step explanation:
One control that can be added to verify on a periodic basis that the files were not modified is Hashing. Hashing involves generating a unique hash value for a file based on its contents. By comparing the hash value generated periodically with the original hash value, you can determine if any modifications have been made to the file.
Another control that can be used is Read-only attributes. By setting the read-only attribute for files, you can prevent them from being modified. Checking the read-only attribute periodically can help verify if any unauthorized modifications have been made.
ACLs (Access Control Lists) can also be used to control file modifications. By assigning appropriate permissions to different users or groups, you can restrict modifications to files and periodically review the ACLs to ensure that unauthorized modifications have not occurred.