The correct statement is option 2: "In 1948, Burma joined the United Nations but refused to denounce China as the aggressor in the Korean War."

Option 1 states that Burma was admitted to the United Nations in 1948 and immediately supported the USA in the Cold War. This statement is incorrect. Burma did join the United Nations in 1948 but it did not immediately support the USA in the Cold War. The statement assumes that Burma took a side in the Cold War, which is not mentioned.

Option 2 correctly states that Burma joined the United Nations in 1948 but refused to denounce China as the aggressor in the Korean War. This statement is correct. Burma`s refusal to denounce China was a significant stance at the time and showed its neutrality in the Korean War.

Since only option 2 is correct, the correct answer is option 2: "2 only."