Which concept does the following sentence describe?

if vietnam were to fall to communism, then all of southeast asia will become communist as well.

a. domino theory

b. containment

c. communism

d. affirmative action

A. domino theory

Explanation:

Vietnam was a communist stronghold in Southeast Asia, having close ties with the Soviet Union. The United States were not very fond of it, and apart from having Vietnam, a country with excellent strategic  location, becoming communist, the real fear was that it will be a domino effect. The US administration was convinced that if Vietnam becomes communist, than the trend will spread in the region of Southeast Asia, thus making a very large region with enormous potential as enemy, while being ally with the Soviets. Because of this fear, the US attacked Vietnam, only to be ashamed by being defeated.

detente translated to english means politics