The correct answer is D) the leader of Korea, Rhee, asked the US for help.
The United States became involved in the Korean conflict because the leader of Korea, Rhee, asked the US for help.
North Korea with the support of the Soviet Union invaded South Korea in June 1950. The United States answered the call of help from South Korea with the aid of other countries. That is when Communist China joined the War supporting North Korea and attacking US troops. Behind North Korea was the Soviet Union that wanted to spread Communism in all the Korean territory, but the United States and its foreign policy of Containment were decided to stop the spread of Communism.