The issue that President Truman was referring to in this address was B. Applying containment policies in Korea
In this excerpt, President Truman is talking about the decision to engage in a military conflict in Korea, which was a key aspect of the United States' policy of containing the spread of communism during the early years of the Cold War.
He is discussing the choice between addressing the issue of communist aggression in a limited area (Korea) now or allowing it to potentially escalate into a much larger conflict later.
The Korean War was indeed a manifestation of the containment policy, as the United States and its allies sought to prevent the spread of communism in Korea and, by extension, in the wider region.