Answer:
C
Step-by-step explanation:
The Korean War came near the end of the 50-year-long conflict called the Cold War between the US and the Soviet Union. The main disagreement was democracy vs communism; the US didn't want communism to spread to the world because of the harmful effects that could follow.
The Korean War broke out when the communist North Korea invaded the democratic South Korea. The US feared that if North Korea won, then South Korea would turn communist, as well, which could potentially lead to the domino effect - which stated that if one country in Southeast Asia fell to communism, others would fall soon after, just like dominoes. So, hoping to prevent such a catastrophe from happening, the US became involved on the South Korean side.
Thus, the answer is C.
Hope this helps!