Which department within the Department of Defense oversees two military branches including the Marines?

The Department of the Navy is responsible for overseeing both the Navy and the Marine Corps, making it the correct choice in this context. The Marine Corps is a unique branch of the military that operates under the Department of the Navy, reflecting its historical ties and operational support. This structure allows for coordinated missions and resource sharing between the Navy and the Marines, crucial for amphibious operations and joint military efforts.

While the Department of Defense is the overarching body that encompasses all military branches, including the Army, Navy, and Air Force, it does not specifically oversee individual branches. The Department of the Army focuses solely on that branch and does not include the Marines. The Department of Homeland Security, however, is primarily concerned with domestic security and does not oversee military branches. This makes the Department of the Navy the definitive answer in this scenario.