The korean war officially began in 1950 when
o south korea arrested and imprisoned thousands of communists.
north korea crossed the 38th parallel and invaded south korea.
o south korea held free elections but north korea did not.
o north korea sought the support of china and the soviet union.

The Korean War officially began in 1950 when North Korea crossed the 38th parallel and invaded South Korea.

Explanation:

1950 was the year in which the "Korean War" started officially and in this year the North Korea crossed the "38th parallel" and attacked on the South Korea. This War was fought between the "North Korea and South Korea" took palace in 1950. The war started on 25th June 1950 and lasted till 27 July 1953.

The North Korea was fighting in this was with the support of China and Soviet Union while South Korea was had the support of United States of America.  

The right answer is when North Korea crossed the 38th parallel and invaded South Korea.

Explanation:

The communist leader Kim Il-sung had been planning the invasion of South Korea to reunify the peninsula by force. His armies finally invaded after getting the approval of Soviet leader Joseph Stalin. Though the United States saw the invasion as the result of a plan of deliberate expansion of communist influence in Asia, actually it was a miscalculation. Stalin and Kim agreed to launch the war believing that the United States would not bother to rescue the South Korean government, because they deemed it as irrelevant - China had just fallen months before into communist hands. The US acted as a matter of principle, it was determined to oppose communist agression in Asia and in the world.