Question

in october of 1950, un forces pushed the north koreans as far north as
pusan.
the yalu river.
the 38th parallel.
seoul.

Explanation:

In October 1950, UN forces advanced North Korean troops to the Yalu River. Pusan is in the south. The 38th parallel was the initial dividing line before the war's expansion. Seoul is also in the south.

Answer:

the Yalu River.