The correct answer is C) Permits the return of cargo to the suction side of the pump. The purpose of the relief valve on a cargo pump is to provide a way for excess cargo to be returned to the suction side of the pump, rather than being discharged into the cargo system. This prevents overpressurization of the cargo system and protects the pump from damage. The relief valve allows the pump to continue operating safely if the discharge is blocked or restricted, by recirculating the excess cargo back to the pump inlet. The other answer options are incorrect because they do not accurately describe the primary function of the cargo pump relief valve. Option A is about tank filling, option B is about vapor removal, and option D is about pump shutdown, which are not the main purpose of the relief valve.