Q.Which one of the following is non aromatic?

Aromaticity and Hückel's Rule

A ring system is aromatic only if it satisfies ALL three conditions of Hückel's rule: (i) the ring is co-planar (or, for a fused/heteroatom system, at least the conjugated ring atoms are); (ii) the pi electrons are COMPLETELY delocalised around the entire ring (no interrupting sp3 atom breaking the conjugation); (iii) the ring contains (4n+2) pi electrons for some integer n = 0, 1, 2, .... All three conditions must hold together -- satisfying the electron count alone, or planarity alone, is not enough.

Worked against the standard examples: BENZENE (planar, 6 fully delocalised pi electrons, 6 = 4(1)+2, n=1) -- aromatic. NAPHTHALENE (planar, 10 pi electrons, 4(2)+2, n=2) -- aromatic. ANTHRACENE (planar, 14 pi electrons, 4(3)+2, n=3) -- aromatic. CYCLOPROPENYL CATION (planar, 2 pi electrons, 4(0)+2, n=0, an integer) -- aromatic, the smallest possible Hückel aromatic system. By contrast, CYCLOPENTADIENE (planar, but only 4 pi electrons AND one sp3 ring carbon breaking full delocalisation) fails on delocalisation -- not aromatic; CYCLOOCTATETRAENE, which is non-planar (tub-shaped) regardless of its electron count, fails on planarity -- not aromatic. …