What does domino theory mean as it applies to vietnam war

The domino theory was the basis for the United States strategy of containment, and the reason for entering the Vietnam War. ... The domino theory basically stated if one new countrywent communist in Asia then it would begin a chain reaction that would cause several more Southeast Asian countries becoming communist

Iwould say a, due to the fact these routes display many things that would be useful and provide explanations for how states are known.