To answer your question, we need to consider the timeline of events related to the Vietnam War:
-
Ho Chi Minh took leadership of the Vietcong: Ho Chi Minh was indeed a leader of the communist movement in Vietnam but he was not specifically the leader of the Vietcong, which became a more prominent force during the Vietnam War. Ho Chi Minh passed away in 1969, while the Vietcong was active during the entire war period.
-
Richard Nixon resigned from office after the Watergate scandal: Nixon resigned on August 8, 1974, which was after the fall of Saigon and after U.S. involvement in the Vietnam War had significantly ended.
-
The Khmer Rouge group invaded Vietnam: The Khmer Rouge, led by Pol Pot, were active in Cambodia and did invade Vietnam, but this occurred after the Vietnam War had effectively ended and the fall of South Vietnam in 1975.
-
A cease-fire allowed U.S. troops to leave Vietnam: This refers to the Paris Peace Accords, signed in January 1973, which led to the withdrawal of U.S. troops from Vietnam.
Based on this information, the correct response indicating an event that occurred after the Vietnam War spread into Vietnam but before Vietnam was united under a communist government would be:
A cease-fire allowed U.S. troops to leave Vietnam.