The correct response is:
The domino theory describes the U.S. policy for stopping the Soviet Union from influencing other countries in which people were staging revolutions and independence movements.
The domino theory was a key component of U.S. foreign policy during the Cold War, suggesting that the spread of communism in one nation could lead to a chain reaction of communist takeovers in adjacent countries. As a result, the U.S. sought to intervene in various revolutions and independence movements to prevent such an outcome, particularly in areas seen as vulnerable to Soviet influence.