Art thou Elias? - This is the Greek way of writing Elijah. The Jews expected that Elijah would appear before the Messiah came. See the notes at Matthew 11:14. They supposed that it would be the real Elijah returned from heaven. In this sense John denied that he was Elijah; but he did not deny that he was the Elias or Elijah which the prophet intended Matthew 3:3, for he immediately proceeds to state John 1:23 that he was sent, as it was predicted that Elijah would be, to prepare the way of the Lord; so that, while he corrected their false notions about Elijah, he so clearly stated to them his true character that they might understand that he was really the one predicted as Elijah.
That prophet - It is possible that the Jews supposed that not only “Elijah” would reappear before the coming of the Messiah, but also “Jeremiah.” See the notes at Matthew 16:14. Some have supposed, however, that this question has reference to the prediction of Moses in Deuteronomy 18:15.