The correct answer is D. A gradual withdrawal of all American troops from Vietnam.
Explanation
Vietnamization was a government policy established by United States President Richard Nixon during the Vietnam War to end US involvement in the Vietnam War through a program to expand US aid to South Vietnam's forces and give them more leading role in the war. At the same time, this program gradually reduced the number of US troops in combat. So, the correct answer is D. A gradual withdrawal of all American troops from Vietnam.