Question

what was the name of the theory that said that if one country in a region fell to communism, others would surely follow?
domino theory
arms race
brinkmanship
containment theory

Explanation:

The domino theory posited that if one country in a region became communist, neighboring countries would also fall to communism, much like dominoes falling in a row. Arms race refers to competition in building weapons. Brinkmanship is a risky policy of pushing a dangerous situation to the brink of disaster to gain an advantage. Containment theory aimed to stop the spread of communism by various means but not with the specific "fall - like - dominoes" concept.

Answer:

A. domino theory