The theory that if Vietnam fell to communism then the rest of Indochina and Southeast Asia would also fall to communism is called the Domino Theory. The correct option among all the options given in the question is the first option.
The Domino Theory was mostly prevalent during the years
1950s to the 1980s. This theory actually predicted that if a country comes
under the influence of communism, then the entire region would fall under the
effect due to Domino effect.
