Which branch operates Military Post Offices (MPOs) serving military personnel?

The correct choice is the Department of Defense (DoD) because it is the primary authority that oversees Military Post Offices (MPOs) which specifically serve military personnel and their families. MPOs provide postal services similar to those offered by civilian post offices but are tailored to meet the unique needs of the military community, especially in operational and deployed environments. The DoD ensures that postal operations are conducted efficiently and securely, supporting the communication needs of service members stationed both domestically and abroad.

The other branches mentioned, while involved in broader military and governmental operations, do not specifically manage or operate these post offices. The United States Postal Service, for example, provides general postal services in the U.S. but does not have direct oversight over MPOs. The Federal Aviation Administration focuses on civil aviation regulations and air traffic management, which does not pertain to postal operations. The National Guard Bureau coordinates and manages the affairs of the National Guard but does not handle military postal services. Therefore, the Department of Defense is clearly responsible for the operations of Military Post Offices.