Final answer:
The Secretary of the Navy is responsible to the SECDEF for the operation and efficiency of the U.S. Navy, overseeing various aspects such as recruitment, equipment, and policy implementation in alignment with national security objectives.
Step-by-step explanation:
The individual who is responsible to the Secretary of Defense (SECDEF) for the operation and efficiency of the United States Navy is the Secretary of the Navy. The Secretary of the Navy is a civilian appointed by the President and is responsible for the conduct of all the affairs of the Department of the Navy, including recruiting, organizing, supplying, equipping, training, mobilizing, and demobilizing. The Secretary of the Navy also oversees the construction, outfitting, and repair of naval ships, equipment, and facilities, and is responsible for the formulation and implementation of policies and programs that are consistent with the national security policies and objectives established by the President and the SECDEF.