JEE Main 2025 Jan 28 Shift 1, Chemistry Q17: Resonance & Mesomeric Effect

Rank by the dominant stabilising factor:

C (tropylium): aromatic, 6 $\pi$ electrons in a planar cyclic system (Hückel $4n+2$ with $n=1$). Aromaticity is the strongest stabiliser, so C is the most stable.

A (Ph${}_3$C${}^{+}$): three phenyl groups delocalise the positive charge through extensive resonance. Many resonance contributors.

B (Ph${}_2$CH${}^{+}$): two phenyl groups; resonance stabilisation present but with fewer canonical structures than A.

D (sec-butyl cation): secondary carbocation stabilised only by hyperconjugation and weak induction.

Combining the hierarchy aromaticity > resonance > hyperconjugation, the order is $C>A>B>D$.

$\therefore$ Option (4).