In october of 1950, un troops pushed far into north korea, which
caused north korea to lose the war badly.
caused south korea to lose the war badly.
led general macarthur to stop his advances.
led china to become involved in the war.

Led China to become involved in the war.

Explanation:

During the Korean War, the United Nations stepped in to support South Korea. By that same time, China and the Soviet Union had advised to not trespassed the 38th parallel because if that was made they would intervene in the war.

By October the UN forces overpass the parallel and that led China to become involved in the war with the help of the Soviet Union.

I hope this answer helps you.