Which of the Following Non-U

Which of the following non-U.S.citizens does not require a nonimmigrant visa?

A) Someone with a green card who is returning to the U.S. after visiting family in his or her home country
B) Someone completing a master's degree at an American university
C) Someone visiting the United States on vacation
D) Someone who was sent to the United States by his or her employer to attend a series of meetings and conferences