Answer:
B. Communist rule in one country would lead to communism in its neighbors.
Step-by-step explanation:
The domino theory was a concept widely accepted and incorporated into Cold War US foreign policy. It was a rationale for American engagement in Korea, in Vietnam, in Latin America (after the Cuban Revolution) and in proxy wars against Soviet satellites in the developing world. It was feared that as in the game, if a domino falls, the other pieces will follow.