A round fruit appears flat because straight parallel strokes were used. What should be done to show volume?

Correct answer: Curve the strokes according to the fruit's form

The governing drawing principle is that contour or form-following strokes describe the direction of a surface. On a round fruit, curved strokes that follow its bulging structure help the eye perceive curvature and volume; straight parallel marks flatten the form. Therefore option D is correct. Erasing, darkening only the outline, or adding dots does not describe the fruit's three-dimensional surface effectively.