The correct answer is D) The United States saw that Ngo Dinh Diem was alienating South Vietnamese citizens.
The United States supported a change in South Vietnamese leadership because the United States saw that Ngo Dinh Diem was alienating South Vietnamese citizens.
US President Richard Nixon established the foreign policy of Vietnamization to reduce the US troops participated in the Vietnam War. He wanted South Vietnam to be more responsible in the war affairs. The US troops would have more participation in training the South Vietnamese soldiers.
After the Tet offense and the bad results for South Vietnam and the American troops, US citizens started to seriously question the participation of the US in Vietnam. People took the streets and protested against the war.
The other options of the question were A) The United States thought that Ngo Dinh Diem had become a communist. B) The United States believed that Ngo Dinh Diem should hold free and fair elections. C) The United States believed that Ngo Dinh Diem did not support American Special Forces.