The shifting of ground combat operations to the south vietnamese and the gradual withdrawal of american troops from south vietnam was called
a. containment
b. the domino theory
c. the truman doctrine
d. vietnamization

The answer is Vietnamization or letter d.  Nixon vowed to withdraw U.S. troops from Vietnam thereby transferring the responsibility of fighting the war to the South Vietnamese army.  Apart from that, he also began negotiations with North Vietnamese.  Combat was focused on covert operations as the Americans eventually left Vietnam allowing the North to unite the South.

answer; overt behavior;