Why did the US want to support South Korea?

The United States came to the aid of South Korea at the head of a United Nations force composed of more than a dozen countries. Communist China joined North Korea in the war in November 1950, unleashing a massive Chinese ground attack against American forces. The Soviet Union also covertly supported North Korea.

Explanation:

In June 1950 communist North Korea invaded South Korea. The United States came to the aid of South Korea at the head of a United Nations force composed of more than a dozen countries.

Communist China joined North Korea in the war in November 1950, unleashing a massive Chinese ground attack against American forces. The Soviet Union also covertly supported North Korea.

After three years of fighting, the war ended in a stalemate with the border between North and South Korea near where it had been at the war’s beginning.

This was the first hot war of the Cold War, and in it the United States demonstrated its continued commitment to containment (the idea that the US would ultimately defeat communism by containing its spread).

The Korean War begins

When Korea was liberated from Japanese control at the end of the Second World War, the United States and the Soviet Union agreed temporarily to divide Korea at the 38th parallel of latitude north of the equator. This division resulted in the formation of two countries: communist North Korea (supported by the Soviets) and South Korea (supported by the United States).  

i would say it is a and b.

hope this