What is the primary purpose of a contour line on a map?

The primary purpose of a contour line on a map is to show relief and elevation. Contour lines connect points of equal elevation, which provides a visual representation of the terrain's shape and slope. By analyzing the spacing and shape of these lines, one can determine whether a hill, valley, or flat area is present, along with the steepness of the terrain. This information is critical for various activities such as planning military operations, hiking, or any outdoor activities that require knowledge of the landscape.

The other options focus on different map features: bodies of water and roads are indicated by other symbols and lines, while boundaries are defined by different types of markings or color codes. Contour lines specifically serve the unique function of illustrating vertical dimensions and elevational changes.