The correct answer is A.
The domino theory was a Cold War policy. It was prominent from the 1950s to 1980s. It compared neighboring countries to perfectly aligned rows of domino, meaning that if the first country falls to communism the second follows as well, and then the third, the fourth etc. It is called the domino effect and it was one of the main arguments used to justify American military involvement in the Vietnam War because of Vietnam's proximity to Corea.
