The Pentagon is the iconic headquarters of the United States Department of Defense, making USA the correct answer. This massive, five-sided building, located in Arlington, Virginia, is a global symbol of American military power and strategic planning. It houses the offices of the Secretary of Defense, the Joint Chiefs of Staff, and various military branches, coordinating national security operations worldwide.
The other options are incorrect because their respective military headquarters are located elsewhere. The UK's Ministry of Defence is primarily in London. Canada's Department of National Defence is situated in Ottawa. Similarly, Russia's Ministry of Defence operates from Moscow. Each nation has its own distinct central command structure, and the Pentagon is uniquely associated with the United States.