Which lines help strengthen the illusion of volume?

Correct answer: Contour lines and cross-contour lines

The correct answer is A: contour lines and cross-contour lines. A contour line follows the outside edge of an object, so it tells us where the form ends. A cross-contour line travels across the surface and follows its curve, bend, or change in direction. When these lines are used thoughtfully, the viewer imagines that the surface turns in space, so a flat drawing can suggest a rounded or solid object. Option A is therefore the strongest answer. Option B is wrong because horizontal and vertical lines mainly communicate orientation; they do not automatically wrap around a form. Option C is wrong because decorative borders and patterns may add beauty but do not necessarily describe three-dimensional construction. Option D is wrong because broken or dotted lines may suggest interruption or texture, but they do not by themselves show the turning surface. Memory cue: a contour outlines a form, while a cross-contour appears to wrap around it; lines that wrap suggest volume.