What does "vertical face" describe?

The term "vertical face" specifically refers to a steep, upright side of a mountain, which aligns with the correct answer. This description emphasizes the degree of steepness and the orientation of the surface, indicating that it is not flat or gently sloping. Vertical faces are often associated with cliffs or sheer rock formations that rise sharply from the surrounding terrain. This characteristic makes them distinct from other types of mountain features, such as flat surfaces, natural bases, or areas with gentle slopes, which would not be described as vertical.