In which form should the value transition be the smoothest?

Correct answer: Sphere

The governing concept is continuous curvature in light and shade. A sphere has no flat faces or sharp corners; its surface turns gradually in every direction. As the surface changes its angle to the light source, the value usually moves through a controlled sequence from highlight to middle tone, halftone, and shadow. Thus option B is correct when the sphere is drawn with a gradual, blended transition. A cube and a rectangular solid have plane changes and clear edges, while a pyramid has distinct triangular planes and sharper value divisions. The exact softness still depends on the light size and material, but among these choices the sphere provides the most naturally continuous transition. Good shading should describe the roundness rather than create separate patches.