Which line crosses a surface and defines changes in the surface of a form?

The correct choice is the cross-contour line, as this type of line follows the form of an object and indicates the three-dimensional changes in its surface. Cross-contour lines move across the surface, showing the ups and downs of the shape, much like contour lines on a map that depict elevation changes. This technique adds a sense of depth and volume to the drawing, helping viewers visualize the object's dimensionality.

In contrast, a horizon line represents the viewer's eye level and does not convey the surface changes of a form. Calligraphic lines are characterized by their fluid, varied thicknesses mainly for expressive purposes, and contour lines outline the edges of shapes but do not traverse the surfaces to show internal variations. Thus, while each option has its own significance in art, cross-contour lines are specifically designed to map out the contours of a form as they reveal its surface changes from a three-dimensional perspective.