Why can a very hard edge look wrong in clouds made with line?

Because clouds usually have soft and blurry edges

Clouds are usually perceived as soft, changing masses rather than objects with sharply cut geometric borders. Their visible edges often fade or vary because cloud droplets are dispersed through air and because light passes through different thicknesses of the cloud. In a line drawing, a very hard, uniform outline can make a cloud appear solid, rigid, or artificial. Softer, broken, varied, or less definite lines better suggest its light and vaporous character.

The correct answer is option C because clouds commonly have soft and blurry-looking edges. The statement that clouds are always square is false, and saying they have no form is also incorrect; clouds do have visible volume and structure, but it is irregular. A hard line is not always best: edge quality should suit the subject. Thus, when drawing clouds with lines, gentle variation and softened boundaries usually communicate their natural appearance more effectively.

This is a Class 10 Art Education question. Chapter: Fundamentals of Visual Arts. Topic: Line.