What is useful for showing a wooden surface in a sketch?

Correct answer: Linear texture

The governing concept is visual texture: artists use marks that resemble the structure of a material. Wood commonly has long grain, so repeated, slightly varied lines can suggest its fibrous direction and surface pattern. Circles alone do not communicate wood grain, white color does not define the texture by itself, and paper cutting is a craft process rather than a sketching mark. Thus option C is correct.