Final answer:
The President of the United States holds the responsibility to command and oversee the Department of Defense as the Commander-in-Chief of the U.S. Armed Forces.
Step-by-step explanation:
The responsibility to execute approved policy, as well as exercising direction, control, and command over the Department of Defense is primarily maintained by the President of the United States. The President is designated as the Commander-in-Chief of the Army and Navy of the United States, as well as the Militia of the several States when they are called into actual service. This role includes the ability to oversee the many elements of the U.S. armed forces, including the Army, Navy, Marine Corps, and Air Force. As such, the correct answer to the question is D. President of the United States.