Solution


Tropylium cation (6 $\pi$e), cyclopropenyl cation (2 $\pi$e) and cyclopentadienyl anion (6 $\pi$e) are planar, fully conjugated and obey Huckel's (4n + 2) rule – aromatic.
Cycloheptatriene has an $sp^3$ ($CH_2$) carbon, so it is not fully conjugated – non-aromatic.

Concept identifying non-aromatic species

Aromaticity – Huckel's Rule in past papers

2 questions from this chapter have appeared across 2 exam years.