Which department oversees the armed forces of the United States?

The Department of Defense is the correct answer because it is the primary federal agency responsible for coordinating and overseeing all agencies and functions related to national security and the armed forces. This department is tasked with ensuring military readiness and making strategic decisions regarding defense policy, military operations, and the resources needed to support the United States military services, including the Army, Navy, Air Force, Marines, and Coast Guard.

In contrast, the other departments have distinct areas of responsibility that do not pertain to military oversight. The Department of the Interior focuses on managing and conserving federal land and natural resources; the Department of Energy handles energy production and nuclear material safety; and the Department of Education is concerned with national policies and federal assistance programs in education. Thus, none of these departments address the oversight and operational needs of the armed forces, reinforcing the unique role of the Department of Defense.