Final answer:
The product of the addition of HCl to cyclopentadiene is cyclopentadienyl chloride, formed by the addition of hydrogen halide to an alkene type of reaction, resulting in a saturated molecule.
Step-by-step explanation:
When hydrochloric acid (HCl) is added to cyclopentadiene, a hydrochloride adduct is formed. In this case, HCl will add across one of the double bonds of cyclopentadiene, resulting in a product where the chlorine atom is bonded to one carbon and the hydrogen atom is bonded to another carbon of the double bond, breaking the π-bond and creating a saturated carbon. The product formed is actually 3-chloro-cyclopentene, or cyclopentadienyl chloride if considering the initial cyclopentadiene molecule as a whole.
Therefore, the correct answer is a) Cyclopentadienyl chloride. This follows the general mechanism for the addition of hydrogen halides to alkenes, similar to the formation of chloroethane when ethyl chloride is produced by the addition of HCl to ethylene.