During the Vietnam War, South Vietnam was primarily supported by the United States. The U.S. provided extensive military, economic, and logistical support to the South Vietnamese government in its fight against the communist forces of North Vietnam and the Viet Cong.
While the Soviet Union and China supported North Vietnam, they did not support South Vietnam. Nazi Germany was not involved in the Vietnam War, as the conflict occurred well after World War II. Therefore, the correct answer regarding who supported South Vietnam is the United States.