What is the capital of the United States?

What does this answer mean?

The capital of the United States is Washington, D.C. "D.C." stands for the District of Columbia.

The Constitution gave Congress the power to govern a district, no more than ten miles square, to serve as the seat of the federal government. In 1790, Congress passed the Residence Act, which chose a site on the Potomac River for the permanent capital and made Philadelphia the temporary capital for ten years. The House of Representatives moved to the new city of Washington in November 1800, meeting in the unfinished Capitol.

Washington, D.C., has been the seat of the federal government ever since, and Congress still meets in the Capitol there.

Don't confuse this question with "What is the capital of your state?" Every state has its own capital, and Washington, D.C., is not part of any state. That's also why D.C. has no senators or governor.

At the interview, say "Washington, D.C."