Why can showing hard flat planes on a sphere look wrong?

Because a sphere has a continuously curved surface, so light and shade change gradually

The governing concept is the relationship between surface geometry and tonal transition. A sphere curves continuously in every direction, so the visible surface turns gradually away from the light. Its modelling normally moves through a smooth sequence of highlight, light, middle tone, core shadow, and reflected light, although the exact pattern depends on the light source and surroundings. Hard, sharply separated planes imply abrupt changes in direction; they can make the object appear faceted, cubic, or polygonal rather than spherical. Option A is correct. Option B is false because a sphere clearly receives light and produces both form shadow and cast shadow. Option C describes a cube, not a sphere. Option D is also false: the sphere is curved around its entire surface, not only on its front-facing portion.

This is a Class 9 Art Education question. Chapter: Elements of Art. Topic: Form.