Which of the following is not aromatic?

Step-by-step solution

For aromaticity: cyclic, planar, fully conjugated, (4n+2) electrons. B (Tropylium cation): 6 electrons ( n=1 ), planar, fully conjugated aromatic. C (Cyclopentadienyl anion): 6 electrons ( n=1 ), planar, fully conjugated aromatic. D (Naphthalene): 10 electrons ( n=2 ), planar, fully conjugated aromatic. A ([10]annulene): Has 10 electrons ( n=2 ), satisfying Hückel's rule, but severe steric hindrance between internal H atoms forces the ring out of planarity not planar not aromatic.