Under which department does the United States Coast Guard serve during peacetime?

The United States Coast Guard operates under the Department of Homeland Security during peacetime. This structure allows the Coast Guard to effectively perform its primary missions, which include search and rescue, law enforcement, environmental protection, and maintaining navigational aids.

While the Coast Guard is a military service, it is unique in that it is not part of the Department of Defense during peacetime. Instead, it serves civilian roles such as protecting the nation's maritime interests and ensuring safe and secure waterways. In times of war or when directed by the President, the Coast Guard can be transferred to the Department of the Navy, but during regular operations and peacetime, its alignment with the Department of Homeland Security highlights its specific focus on domestic security and support functions.

The other options, such as the Department of Defense, Department of Transportation, and Department of Commerce, do not reflect the Coast Guard's operational structure during peacetime.