Final answer:
The correct answer is option c) The division of North and South Korea along the 38th parallel and a lasting ceasefire.
Step-by-step explanation:
The most important result of US participation in the Korean War was c) The division of North and South Korea along the 38th parallel and a lasting ceasefire.
At the end of World War II, Korea was divided along the 38th parallel, with the Soviet Union administering the north and the United States administering the south. The Korean War, which began in 1950, ended in a stalemate and resulted in the division of North and South Korea along the same 38th parallel. A ceasefire was signed in 1953, which has remained in effect since then.
The other options listed in the question are incorrect. North and South Korea have not been reunified, and there is no democratic government in North Korea. The United States did not establish itself as a colonial power in Korea.