Which of the indicated carbon atoms is the least likely site of protonation in the conjugated alkene shown below?

Step-by-step solution

The given molecule is a substituted fulvene, which contains a cyclopentadiene ring with an exocyclic double bond. To find the least likely site of protonation, we must evaluate the stability of the carbocation formed after a proton ( H^+ ) is added to each indicated site. The least likely site will form the most unstable carbocation. If protonation occurs at site 'a' (the exocyclic CH₂ carbon), the electrons from the exocyclic double bond are used to form a bond with the proton. This leaves a positive charge on the