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 the North Koreans back to the 38th parallel. Pusan is in the south - not a location they were pushed back to. The Yalu River is further north and was reached later. Seoul is also not the location in this context. The 38th parallel was the pre - war border and the location relevant to this push.

Answer:

the 38th parallel