Elsenhower's that the fall of one nation to communism would lead to others

a. liberation
b. brinkmanship
c. the domino theory
d. convert action

The right answer is:  C. The domino theory.

Explanation:

It was an accepted idea or notion in the Washingtonian establishment during the Cold War that if one nation was allowed to fall under Communist rule, then neighboring nations would follow suit. This was a rationale for American engagement in Korea and Vietnam, and even in Central America in the 1980s. That idea perfectly fitted into the framework of the US containment doctrine.

c or d. good luck !