Which of the following compounds is aromatic?

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(B) compound is aromatic if it follows $H$ückel's rule: it must be cyclic,planar,fully conjugated,and contain $(4n+2) \pi$ electrons,where $n$ is an i

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(B) compound is aromatic if it follows $H$ückel's rule: it must be cyclic,planar,fully conjugated,and contain $(4n+2) \pi$ electrons,where $n$ is an integer $(0, 1, 2, ...)$.$1$. Option $A$: This is the pyrrolium cation. It has $4 \pi$ electrons (from two double bonds). It is anti-aromatic.$2$. Option $B$: This is an imidazolium-like cation. The nitrogen atom with the lone pair participates in the aromatic sextet. The ring has $6 \pi$ electrons ($4$ from two double bonds and $2$ from the lone pair on the nitrogen atom). It is planar and cyclic. Thus,it is aromatic.$3$. Option $C$: This is a pyrylium-like cation. It is not fully conjugated in a way that satisfies the $(4n+2)$ rule for the entire ring system.$4$. Option $D$: Styrene is a benzene derivative. While the benzene ring is aromatic,the question asks for the compound as a whole. However,in the context of these specific choices,the imidazolium-type cation $(B)$ is a classic example of a heterocyclic aromatic system.

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