Explanation: In a fully charged lead-acid storage cell, the negative plate is composed of pure spongy lead (Pb). The "spongy" texture provides a large surface area, which is crucial for efficient chemical reactions during charging and discharging. Conversely, the positive plate in a charged cell is made of lead peroxide (PbO2). During discharge, both plates react with the sulfuric acid electrolyte to form lead sulfate (PbSO4). Therefore, option B is incorrect because lead peroxide forms the positive plate. Option C is incorrect as the plates are distinct materials, not a compound of both in their charged state.