What is the standard voltage for AC power in the United States?

Correct answer: 120V

Option D is correct because the standard nominal voltage for AC power in the United States is 120 volts (at 60 Hz), as defined by ANSI C84.1, and this is what residential outlets deliver to consumer electronics and computer power supplies. Option A is wrong because 100V is the standard used in Japan, not the United States. Option B is wrong because 110V is an informal or older colloquial reference sometimes used in conversation, but the actual standard nominal voltage is 120V, not 110V. Option C is wrong because 240V is used in many other countries (and for high-power appliances like dryers and ovens in the US on a split-phase circuit), but it is not the standard single-phase outlet voltage in the United States.