Answer:
Option D, The United States committed millions of troops to the conflict in Vietnam because it feared that surrounding countries would fall to communism if Vietnam did, is the right answer.
Step-by-step explanation:
A theory that became prominent during the period between the 1950s to the 1980s came to be known as the Domino Theory. According to this theory, in the event that Vietnam is influenced by communism, then the encompassing countries would also follow in a domino effect. This theory was explained by Dwight D. Eisenhower, the then President of the United States, on 7th April 1954, to News Conference. On the basis of this theory, The United States decided to get involved in the Vietnam Conflict.