Why is an outer contour alone insufficient when drawing tree bark?

Correct answer: Because surface direction, cracks, and tones together describe texture

The governing concept is surface texture and form description. An outer contour records only the silhouette; it cannot show the bark’s ridges, cracks, direction, uneven planes, or tonal variation across the surface. Therefore D is correct. Colour alone, inability to measure height, and an assumption about a dark background do not explain how bark texture is represented. Controlled marks and values are needed inside the contour.