The domino theory was a theory prominent from the 1950s to the 1980s that posited that if one country in a region came under the influence of communism, then the surrounding countries would follow in a domino effect.
A domino effect or chain reaction is the cumulative effect produced when one event sets off a chain of similar events. The term is best known as a mechanical effect and is used as an analogy to a falling row of dominoes.
So the answer is the last one, “It was the idea that if one nation fell to communism, others would soon follow the example.”