The Cattle - 6:9
If We had made him an angel, We would have sent him in the form of a man and left him in his present doubts.
What were their doubts?
Explained in Detail/41:14
"Messengers went to them from before them and behind them, saying: "Do not serve or serve other than Allah". They said, "If our Lord had willed, He could have sent down an angel. Therefore we do not believe in the message you bring"."
Prophet Abraham/14:9
"Has not reached you the news of those who were before you, of Noah and the people of Aad and Thamud and those who came after them, whom only Allah knows? Their messengers went to them with clear proofs, but they despised them and said: "We disbelieve in what you have brought and we have doubts and misgivings about the message to which you call us"."