Step 1: Recall the Jones major criteria. The major criteria for acute rheumatic fever are carditis, polyarthritis, chorea, erythema marginatum, and subcutaneous nodules.
Step 2: Spot the odd one out. Carditis, chorea, and erythema marginatum are all major criteria. Polyarthralgia (joint pain without inflammation) is a minor criterion, not a major one. So polyarthralgia is the answer to a question asking what is not a major criterion.
Step 3: Note the distinction. Polyarthritis (with inflammation) is a major criterion, but polyarthralgia (pain only) is minor. The wording therefore separates option a from the other three, which are major manifestations.