Among the given species the Resonance stabilised carbocations are :

Solution

Chemistry ma’am · 2 steps

Check the carbon right next to the positive one: a π bond there means resonance.

1. The test. A carbocation is resonance stabilised only if its empty p orbital lines up with a π bond on the directly attached carbon. The charge can then spread onto another carbon. If even one sp³ carbon sits in between, the π electrons cannot reach the empty orbital.

   Species	What is attached to the C⁺	Resonance?
   (A) benzyl	a benzene ring carbon, part of the aromatic π system	yes, the charge spreads to the ortho and para carbons
   (B) allyl, CH₂=CH-CH₂⁺	a C=C carbon	yes, the charge is shared by both end carbons
   (C) cyclohexenylmethyl	an sp³ ring carbon, and the C=C starts one carbon further away	no, it is not conjugated
   (D) vinyl, CH₃COCH₂CH=CH⁺	the charge sits on a doubly bonded carbon itself, and its empty orbital is perpendicular to the π bond	no, and the C=O is cut off by a CH₂

2. So. Only (A) and (B) have a π bond conjugated with the empty p orbital, so the answer is (c).

The idea to keep. For resonance in a carbocation, the π bond must be on the carbon directly bonded to C⁺. A double bond one carbon away gives no resonance, and a vinyl cation cannot use its own π bond.

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