Identify the adverb clause and the word it modifies.
gary plays the trombone better than he did last year

The correct answer is "better than he did last year'. This is an adverbial clause that modifies the verb plays. It shows the manner in which he plays and the manner is "better than he did last year'. The trombone is an object and Gary is the subject, leaving this to be the adverbial clause of manner.

It’s the first one.