The correct order of stability of given carbocation is :

Solution

Chemistry ma’am · 4 steps

Ask two things: can the positive charge spread out, and how electronegative is the carbon that holds it?

1. Resonance wins. In A (benzyl), the empty p orbital of CH₂⁺ lines up with the ring's π system. The positive charge spreads over three ring carbons, so A is the most stable of the four.
2. Hyperconjugation next. In C (ethyl), the positive carbon is sp². The three C–H bonds of the CH₃ group feed electron density into its empty p orbital, which gives some stabilisation.

   Carbocation	Positive carbon	What helps or hurts	Stability
   A, C₆H₅CH₂⁺	sp²	resonance with the ring	highest
   C, CH₃CH₂⁺	sp²	hyperconjugation (3 C–H)	second
   B, CH₂=CH⁺	sp	no resonance, charge on an sp carbon	third
   D, HC≡C⁺	sp, triple-bonded	no help, carbon with the highest s-character	lowest

3. Vinyl and ethynyl. In B and D the double or triple bond does not spread the charge. The π bond is perpendicular to the empty orbital, so it cannot conjugate with it. More s-character makes a carbon more electronegative, and an electronegative carbon holds a positive charge badly. The ethynyl carbon in D is the worst at this, so D is the least stable.
4. So. The order is A > C > B > D, which is option (a).

The idea to keep. Carbocation stability: resonance beats hyperconjugation. A positive charge on an sp² carbon is far better than one on an sp carbon. Vinyl and ethynyl cations are among the least stable you will meet.

Answer from the NTA final answer key. Solution by PrepWiser.