Which of these was used to justify us support for south vietnam

The Domino theory.

Explanation:

A cold war policy that a communist government could lead to influence and control of other neighboring nations by communists like a row of perfectly aligned dominos would fall and this theory was called the domino theory. This theory was used by the United States government to Justify its support to a non-communist dictator of South Vietnam. This theory crumbled when communism failed to show strong foot in South eat Asia.

d. migration of french settlers into english colonies