The correct answer is:
D. the rest of Southeast Asia
Explanation:
1. The domino theory was a foreign policy concept during the Cold War era. It suggested that if one country in a region came under the influence of communism, then neighboring countries would also follow suit like a row of falling dominoes.
2. In the context of the Vietnam War, the domino theory was used to argue that if Vietnam fell to communism, then the rest of Southeast Asia would also fall to communism one after another.
3. This idea was a significant factor in the United States' involvement in the Vietnam War, as they feared the spread of communism in the region and aimed to prevent the domino effect by supporting South Vietnam against the communist North Vietnam.
Therefore, the domino theory highlighted the concern that if Vietnam fell to communism, it would lead to the spread of communism to the rest of Southeast Asia.
