JEE Main 2025 Jan 24 Shift 1, Chemistry Q4: Resonance & Mesomeric Effect

The cation is stabilised by donor effects from groups in conjugation with the empty p-orbital. A $+M$ donor placed on the vinyl carbon of an allylic cation can deliver an oxygen (or other heteroatom) lone pair directly into the $\pi$-system, dispersing the positive charge onto the heteroatom.

Option (2) has $-\mathrm{OC}{\mathrm{H}}_3$ on the vinyl carbon, so the oxygen lone pair is in full conjugation with the cation, producing strong $+M$ stabilisation.

Option (1) places the methoxy on an sp${}^3$ carbon beyond the $\pi$-system, so only inductive ($+I$) donation reaches the cation $-$ much weaker.

Option (3) has the oxygen tied up by an electron-withdrawing acyl group, dramatically reducing its donor strength.

Option (4) has F on the vinyl carbon: F is a $+M$ donor but a strong $-I$ withdrawer, and its 2p$-$2p overlap with carbon is poor compared with O.

$\therefore$ Option (2) is the most stable.