Three astronauts manned each crewed Apollo mission.
The standard Apollo crew consisted of a commander, a command-module pilot, and a lunar-module pilot. This three-person arrangement allowed one astronaut to remain in lunar orbit while two descended to the Moon’s surface during landing missions. On Apollo 11, for example, Neil Armstrong and Buzz Aldrin landed in the lunar module, while Michael Collins stayed aboard the command module Columbia.
The crew structure was also used on missions that did not land. Apollo 7 tested the command and service module in Earth orbit with Walter Schirra, Donn Eisele, and Walter Cunningham. Apollo 8 carried Frank Borman, James Lovell, and William Anders around the Moon without a lunar landing.
Strictly speaking, not every numbered Apollo mission was crewed: Apollo 1 never launched, and Apollo 4, 5, and 6 were uncrewed test flights. The quiz’s wording refers to the manned Apollo missions, for which three astronauts was the standard crew size.