Final answer:
Use a password management tool to ensure that a service account, like the one used to run AD FS, is changed periodically.
Step-by-step explanation:
The correct option to ensure that a service account, like the one used to run AD FS (Active Directory Federation Services), is changed periodically is option C) Use a password management tool. Password management tools allow for the automated rotation of passwords at specified intervals, ensuring that passwords are regularly changed without manual intervention. This helps enhance security and reduce the risk of unauthorized access to the service account.