JEE Main · 2021 · 16 Mar 2021 · Shift 1 · Hydrocarbons

A ring is aromatic when it is planar, cyclic, completely conjugated and holds $(4n+2)$ $\pi$ electrons.
Benzene has six $\pi$ electrons spread over six $s{p}^2$ carbons and meets every condition.
The cyclopentadienyl anion puts the carbanion lone pair into a $p$ orbital that joins the two double bonds, so six $\pi$ electrons circulate over five $s{p}^2$ carbons and Huckel's rule is again satisfied.
The cyclopentadienyl cation has an empty $p$ orbital and only four $\pi$ electrons in the cyclic conjugation, which makes it antiaromatic.
In 5-methylenecyclohexa-1,3-diene the third double bond points out of the ring and one ring carbon remains $s{p}^3$, so the conjugation around the ring is broken and the compound is non-aromatic.
The aromatic compounds are benzene and the cyclopentadienyl anion.