Which center processes incoming and outgoing letters and packages for military installations?

The Official Mail Center is responsible for processing incoming and outgoing letters and packages specifically for military installations. It serves as a centralized hub for controlling and managing official correspondence, ensuring that mail is properly distributed and handled in accordance with military regulations. This center is crucial for maintaining effective communication within various military branches and ensuring that official documents are processed efficiently.

In the context of the other options, while Military Post Offices also handle mail, they primarily focus on delivering mail to service members and their families as well as managing postal services, rather than exclusively processing official correspondence. Aerial Mail Terminals are involved in the transportation of mail, particularly by air, but they do not directly deal with the processing of incoming and outgoing letters and packages on installations. The Mail Control Activity deals more with the regulation and accountability of mail rather than the processing itself, making the Official Mail Center the most accurate choice for this particular function within military installations.