What is the benefit of curving shading lines along the surface of an object?

Correct answer: The roundness of the form becomes clear

This technique is called contour or form-following shading. The artist curves the pencil marks in the same direction as the object’s surface, so the marks describe its volume instead of appearing as unrelated stripes. On a sphere or cylinder, this helps the viewer understand the rounded form and its three-dimensional quality. Thus option D is correct; the method does not straighten paper, add colour, or make the lines disappear.