Mark 12:15

Shall we give, or shall we not give? But he, knowing their hypocrisy, said unto them, Why make ye trial of me? bring me a denarius, that I may see it.

Shall we give, or shall we not give ... has the effect of "Give us a plain, Yes or No." Jesus asked them to bring him a denarius, the type of coin used in paying the poll tax, the same having an image of Caesar upon one side and of Caesar's mother on the other side, the images being particularly obnoxious to the Jews. There was also an inscription which tacitly acknowledged the divinity of Caesar, also an abomination to Israel. The Lord did not ask for the coin for the purpose of finding out what was upon it, but for the purpose of exposing the fact that they had it in their possession and were using it as coin of the realm.