Which element is most helpful in showing the roundness of a sphere?

Correct answer: A gradual transition from light to dark tones

A sphere has a continuously curved surface, so its light changes gradually across the form. A drawing becomes convincing when the highlight passes smoothly through middle tones into the shadow, producing a rounded three-dimensional effect. A thick outline only indicates the boundary, and a contact shadow only shows where the sphere touches the ground. Uniform black removes tonal information. Hence, option C is correct.