Explanation

(c) Electric field at any interior point of a metallic sphere is zero. When a test charge $q_{0}$ is moved along the equipotential surface, it follows that E must be perpendicular to the surface at every point, so that the electric force $q_{0}\mathbf{E}$ will always be perpendicular to the displacement of a charge moving on the surface, so work done by the electric force is zero.