Well, this was called the "Korean War", because North Korea was invading South Korea in the mission of spreading Communism, which was the Soviet Union's main order, (the Soviet Union was determined to spread communism to other countries), and the United States was AGAINST the spread of Communism, so in order for the Soviet Union to NOT help North Korea, the U.S. pitched in to help South Korea.
Therefore, your answer is C., A need to keep the Soviet Union out of Asia.
=)
