Which solution should be implemented to secure a domain controller that cannot be physically secured?

Implementing a Read-Only Domain Controller (RODC) is an effective solution for securing a domain controller that cannot be physically secured. An RODC is a variation of a standard domain controller that contains a read-only copy of the Active Directory database. This setup minimizes the risk associated with physical breaches because even if the RODC is compromised, the attacker cannot modify the Active Directory data.

RODCs are particularly useful in scenarios where the physical security of the domain controller is in question or where the location is known to be less secure. In such cases, having an RODC helps to reduce the risk of elevation of privileges and prevents unauthorized changes to the directory information. Additionally, RODCs can be configured to cache credentials for users, which enhances performance while still maintaining a level of security since it's impossible to modify the stored data.

The other choices do not provide the specific benefit of securing a domain controller in a physically insecure environment. Active Directory Certificate Services focuses on managing digital certificates and does not directly address the risks associated with physical access to a domain controller. Database Mirroring is a high-availability feature that does not involve securing a domain controller itself. Finally, a File Sharing Service is unrelated to the specific security concerns regarding domain controllers.