The correct answer is "D".
The Korean War was a conflict fought between North Korea, which had the support of China and the Soviet Union, and South Korea, which had the support of the United Nations.
Korea was split into 2 sovereign states in 1948. The North Part followed a Communist model and the South Part was Anti Communist. Both leaders from these states claimed sovereignty over the other territory, which led to to the outbreak of the War on June 25, 1950.
The War ended in 1953 with the signature of an Armistice on July 27th, 1953 and both territories remained as separate countries.