The correct answer is 2. The last leader of South Vietnam was Duong Van Minh.
Duong Van Minh was president of South Vietnam. He served as president in 1964 and again in 1975, being the last to take office after the fall of Saigon.
On April 28, 1975, the National Assembly of South Vietnam transferred power to Minh, following the resignation of Tran Van Huong. After two days, North Vietnamese forces entered Saigon and Minh signed the surrender.