Which of the following is heterocyclic aromatic compound?

A compound is considered aromatic if it is cyclic, planar, completely conjugated, and satisfies Hückel's rule by having $(4n + 2)\pi$ electrons.

Let us examine each option:

Option (1): The structure is a pyrylium ion. It is a six-membered heterocyclic ring containing an oxygen atom. The oxygen atom carries a positive charge and is $sp^2$ hybridized. There are three double bonds in the ring, contributing $6\pi$ electrons. The molecule is cyclic, planar, fully conjugated, and has $6\pi$ electrons (n=1$in Hückel's rule). Therefore, it is a heterocyclic aromatic compound.<br><br>Option (2): The structure is tetrahydropyran. It is a heterocyclic compound but lacks any double bonds. It is non-planar and not conjugated. Therefore, it is non-aromatic.<br><br>Option (3): The structure is 4H$-pyran. It contains two double bonds (4\pi$electrons). One of the carbon atoms in the ring is$sp^3$hybridized, breaking the continuous conjugation. Therefore, it is non-aromatic.<br><br>Option (4): The structure is phenol. It consists of a benzene ring with a hydroxyl group attached. The ring is carbocyclic (containing only carbon atoms in the ring), not heterocyclic. While phenol is an aromatic compound, it is not a heterocyclic aromatic compound. Answer :$