Answer: C. Richard Nixon
Context/detail:
Richard Nixon came into office as President in January, 1969. By that time the war in Vietnam involved hundreds of thousands of American troops and over 30,000 American lives had already been lost in the war. The war had become increasingly unpopular with the American people. In November, 1969, President Nixon gave a speech which announced his Vietnamization policy, which emphasized that the United States must empower South Vietnamese forces to assume more combat duties.
By the time the US was shifting emphasis to this sort of policy, it was too late to stave off the victory of the North Vietnamese and Viet Cong forces. The US eventually withdrew its forces from Vietnam in 1973, and by 1975, Saigon (in South Vietnam) fell to the North Vietnamese communist forces.