The correct answer is “Saigon”.
Saigon fell the Communist on April 30, 1975.
Saigon was the capital of South Vietnam. On that date, the Viet Cong and the People’s Army of Vietnam invaded and captured Saigon. This meant the ending of the war in Vietnam and started the process of unification of North and South Vietnam into one country. The new named elected for the country was the Socialist Republic of Vietnam.