Actually, none of the multiple choices are correct. The first American president to send combat troops to Viet Nam was Harry S. Truman, in 1945. The Army personnel sent were a team from the Office of Strategic Services, the forerunner of both the CIA and the Army's Special Forces.
If what the question means is troops after the partition of the country into North and South Viet Nam, that's different. If your teacher means regular American military units, both combat and support, that would be Johnson in 1965. But from 1946 (in the Greek-Albanian conflict) through mid-1983 American military advisors were considered combat troops, so the answer would be Eisenhower, in 1955. This is an example of a situation in which the teacher's opinion and definition dictates the "correct" answer.
Military Assistance Command Groups included headquarters and administrative personnel, but the soldiers who trained indigenous troops were expected as a part of their regular duties to go into combat with the troops they advised. They were termed "advisors" because they advised the troop's commander in the field in combat. In El Salvador in 1983 the term advisor was dropped in favor of "trainer", and the American trainers were no longer allowed an actual combat role.
Code-named "Deer Team," the 1945 group trained the cadre of what later became the Viet Nam People's Army, what Americans called the "Viet Minh" in the late 1940s and '50s, and the North Vietnamese Army later. Deer Team members led the Vietnamese against Japanese troops, and for some months after the end of WW II against the French who were reestablishing colonial control.