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 pushed North Koreans as far north as the Yalu River. Pusan was a southern city in South Korea. The 38th parallel was the initial division line between North and South Korea before the war's expansion. Seoul is the capital of South Korea. The Yalu River forms the border between North Korea and China.

Answer:

the Yalu River.