This is a Diels-Alder reaction between cyclopentadiene (diene) and maleic anhydride (dienophile). The reaction proceeds via a concerted [4+2] cycloaddition mechanism, forming a bicyclic adduct. Maleic anhydride adds to the less hindered endo face of cyclopentadiene, resulting in the endo product due to secondary orbital interactions which favor its formation over the exo isomer.

The product is the endo adduct, which corresponds to option (C).