The correct answer is A.
The draft was the system used by the US army in order to fill in the different positions that could not be covered by volunteers. It was used in six different wars: the American Revolution, the American Civil War, WWI, WWII, and two proxy wars during the Cold war era: Korean and Vietnam Wars.
During the Vietnam War, two thirds of the US military forces were consituted by volunteers and the rest were selected using the draft. In the Vietnam conflict, which has been the last in which the draft was used, there was a growing number of exemptions and deferements that were granted for college students who were eligible for the draft.