1963 the united states stopped its support for the leader of south vietnam because

The government of Diem in South Vietnam was a republic, but in fact he had absolute power over the affairs of the State in the RVN. The judicial branch was appointed by Diem, now as president and no longer as Prime Minister. It was a National Assembly composed of a Senate and Congress, but it did not have many influences on the policies of the RVN. Diem governed until 1963 with the rule of force, and many times took advantage of the NRSV to defeat his opposition as in the Buddhist crisis of 1963. Many of the acts of the government of Diem were not made from their state powers, they were made according to a network of groups loyal to him such as the Can Lao Party, several secret polices, and the support he had in the Catholic Church in Vietnam.

This unpopular situation, coupled with the fact that communism had been infiltrating the South Vietnamese territories for months without any effective opposition, led the United States to demand from Diem government reforms to rectify these errors. When Diem refused to establish reforms, the United States withdrew his support and Diem was overthrown and assassinated in 1963.

the answer is b just took the test