What characterizes a contour line in art?

A contour line in art is characterized by its function to define the edges and outlines of forms within a composition. This line serves as a visual boundary, indicating the shape and structure of objects, both in two-dimensional and three-dimensional works. By providing a clear delineation of an object's outline, contour lines help the viewer understand the form and volume of the subject being depicted.

In contrast, the other options describe different aspects of drawing or artistic expression. A dynamic and fluid line is more indicative of gesture drawing, which emphasizes movement and emotional expression rather than precise outline. Textured patterns convey a sense of surface quality and depth in a different manner, and shading techniques work to create tonal variations, adding drama and dimensionality without serving the specific function of outlining forms. Thus, option A captures the essence of what a contour line represents in art.