The Korean War officially began in 1950 when South Korea arrested and imprisoned thousands of communists. North Korea crossed the 38th parallel and invaded South Korea. South Korea held free elections but North Korea did not. North Korea sought the support of China and the Soviet Union.

Correct answer:

North Korea crossed the 38th parallel and invaded South Korea.

More detail:

At the end of World War II, Korea was divided between an area in the north that was under Soviet influence and in the south under American influence. When 90,000 North Korean troops crossed the 38th Parallel and attacked South Korea on June 25, 1950, US President Harry Truman ordered American troops moved from Japan to South Korea to protect the South against communist aggression. The United Nations also went into action. An emergency session of the UN Security Council was held - - from which the Soviet Union was absent because the USSR was boycotting the UN over the exclusion of communist China from the organization. Truman announced to the American people that he was authorizing sending US troops to prevent South Korea from being overtaken by communism. The UN Security Council met again and approved a US resolution approving the use of force against North Korea. Military forces in the Pacific theater, based in Japan, were deployed in the effort. There was no formal declaration of war by the US Congress, but Congress did vote to extend the draft and also authorized the president to call up military reserve personnel for duty.

B: North Korea crossed the 38th parallel and invaded South Korea.