Which of the following is NOT a type of contour line?

Contour lines are used in topographic maps to represent elevation changes on the terrain. They connect points of equal elevation, allowing readers to visualize the shape and steepness of the land.

Index contour lines are the thicker lines that are labeled with elevation values, making them easier to identify. Intermediate contour lines occur between these index lines and represent additional elevation changes but are not labeled. Supplementary contour lines, used in some contexts, assist in conveying further detail about elevations in areas of very steep terrain but are less commonly referenced.

Horizontal lines, however, do not fit into the established categories of contour lines. While they may represent level surfaces in some contexts, they do not convey changes in elevation, which is the purpose of contour lines. Therefore, identifying horizontal as the option that does not pertain to the categories of contour lines is correct, as it does not contribute to the understanding of elevation changes in a topographic context.