Which one of the following is more reactive towards HBr

detailed solution

Correct option is A

The reactivity in the reactions of the given compounds with HBr explained on the basis of stability of carbocations. Greater the stability of carbocation higher will be the its reactivity. In acidic medium a very stable carbocation is formed as illustrated below.Triphenyl carbocation more stable due to a greater number of resonance structuresThe remaining compounds carbocation will also have stability by resonance/hyper conjugative effect but less stable than Ph3C+.