Which united states foreign policy action best illustrates the concept of containment?

You didn't provide a list of choices, but I'll provide a key example:  The US intervention in Korea.  In Korea, when communist forces from the North invaded the South in 1950, the United States intervened (with United Nations backing), battled to keep South Korea free and democratic, and were able to do so.  The Soviet Union did not play an active, open role in that war, but did covertly supply material services to the North Korean side, as well as Soviet pilots and aircraft.  Keeping communism from spreading in Korea was a way the US was practicing its policy of containment over against Soviet communist influence.