Chemistry - GOC Question with Solution | TestHub

Protonation at b breaks the a=b pi bond and initially leaves positive charge at a. That charge can delocalize through the other ring double bond and the exocyclic double bond, giving an extended pentadienyl-type cation with a contributor bearing charge at the tertiary exocyclic carbon d. Protonation at a gives a shorter allylic path; protonation at c isolates a tertiary cation, and at d gives a disfavored ring cation. The preferred site is b, option $\mathrm{B}$.