The statement that is generally considered a success of the Korean War is: Communism was contained to its prewar borders.
While the other statements may have elements of truth or are related to the aftermath of the war, the primary geopolitical outcome recognized as a success for the United States and its allies was the containment of communism within North Korea, preventing it from spreading to South Korea. The Korean Peninsula remained divided along the 38th parallel, which effectively halted the spread of communism into South Korea.