What may be lacking if the outer sides of a rounded vessel are drawn perfectly parallel from top to bottom?

Correct answer: The actual widening and narrowing of the form

The governing concept is accurate contour observation. A rounded vessel often widens, narrows, or changes its profile between the rim, middle, and base; its sides are not necessarily straight parallel lines. Drawing them perfectly parallel removes these observed changes, so the vessel loses its characteristic widening and narrowing and may look cylindrical or inaccurate. Local tone, cast shadow, and texture are separate features and cannot correct a wrong outer profile. Hence option C is correct.