JEE Main · 2019 · 09 Jan 2019 · Shift 2 · General Organic Chemistry

Aromaticity needs a planar, cyclic, fully conjugated ring with $(4n+2)$ $\pi$ electrons. Pyrrole and pyridine each reach a sextet of $\pi$ electrons and are aromatic, and the cyclopentadienyl anion with its six $\pi$ electrons is aromatic too. The cyclobutadiene-type cation, however, has only four $\pi$ electrons -- a $4n$ count -- so it is antiaromatic, and is the one that is not aromatic.