The correct answer is D. The United States got involved in the Vietnam War to stop communism from spreading to South Vietnam.
The conflict began with an attempt to unify the two Vietnam in a single coalition government between nationalists, communists and neutrals. But it failed because of the armed uprising of several communist groups united under the National Liberation Front, Viet Cong, quickly supported by the then Soviet Union and Mao's China. Initially Saigon was losing ground.
With the massive entry of the United States, which got involved to prevent the spread of communism in Southern Asia, part of what was lost was recovered. But after the events of 1968, the gradual withdrawal of US troops and the signing of the Paris Peace Accords in 1973 began, after which South Vietnam fought alone against the Army of the Democratic Republic of Vietnam until its own defeat and the subsequent reunification of the country on July 2, 1976 as the Socialist Republic of Vietnam.