Explanation: In a fully charged lead-acid storage cell, the positive plate is composed of lead peroxide (PbO2), also known as lead dioxide. This material gives the positive plate a distinctive dark brown color. Here's why the other options are incorrect: * **A) Is composed of pure spongy lead:** This describes the *negative* plate of a fully charged lead-acid cell, which is typically grey. * **C) Is composed of a compound of pure spongy lead and lead peroxide:** The positive plate is primarily lead peroxide; it does not contain pure spongy lead. The two materials are found on separate plates within the cell.