Question

The United States "wound down" its war involvement by transferring fighting from American to South Vietnamese troops, a process that was called

Answer 1

Vietnamization--This process transferred military control back to the South Vietnamese. 

Under the Nixon administration, the US began moving the Vietnam war into the hands of the Vietnamese. This process also allowed from removal of US troops from Vietnam. 

Answer 2

That process was called VIETNAMIZATION.

President Nixon was the one who instituted the 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.