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.
This theory posited that the spread of communism in one nation could lead to a chain reaction, or "domino effect," causing neighboring countries to also fall to communism. As a result, the U.S. and its allies sought to support anti-communist governments and movements during the Cold War to prevent the spread of Soviet influence.