What structure is common to all aromatic hydrocarbons?

Question

You must be referring to the benzene ring, either separately or in combination with alkyl groups or other aromatic structures.

UM.. SORRY DrBob222 BUT i THINK IT IS JUST THE benzene!

i DID SOME REASEARCH SO MAYBE I AM WRONG WHCIH ONE IS RIGHT?

The benzene ring is common. I mentioned the other part to emphasize that methyl benzene (tolune), dimethyl benzene (xylene) as well as fused benzene rings are also aromatic. I'm pleased to hear you did some research. To be truthful about it, you would do better to do the research FIRST, then post questions you have as a result of that. I'm not trying to discourage you from posting questions here for we want you to do that but I am suggesting that you try to find the answer yourself, then post questions you have as a result of that search. You will learn more and retain more information that way. I hope this helps clear up any confusion.