During peacetime, the US Coast Guard serves under the ______________, but in wartime it becomes part of the ______________.

The correct answer highlights the dual role of the Coast Guard, which operates under the Department of Homeland Security during peacetime, managing various maritime safety, security, and environmental protection missions. In times of war, the Coast Guard is transferred to the Department of the Navy, allowing it to integrate with naval forces for wartime operations. This structure reflects the unique position of the Coast Guard, which plays vital roles in both civil and military operations, demonstrating flexibility and the ability to adapt to different operational requirements depending on the national context.

The other options do not accurately represent the current operational structure and command authority of the Coast Guard. The Coast Guard has never served directly under the Department of Defense in peacetime, nor is it a part of the Army or the Department of Justice. These inaccuracies emphasize the importance of understanding the specific governmental departments that control various military and civilian agencies in the United States.