A compound with molecular formula C4H4O has all the four carbon atoms and the oxygen atom in the ring. It also has two alternate double bonds. The compound is

Heterocyclic means, the ring is made up of by carbons and one (or) more hetero atoms.According to Huckels Rule,Monocyclic, planar ,conjugated polyenes having   delocalised electrons are said to be AROMATIC.The above compound is aromatic and heterocyclic.