What type of terrain is marked by flags or horizontal prisms?

The option indicating rugged or unstable terrain is marked by flags or horizontal prisms because these markers serve as indicators for areas that may pose difficulties for navigation or require caution during operations. Such terrain can be characterized by uneven surfaces, potential for rock falls, or other instability factors, making it essential for pilots or navigators to be aware of these hazards.

Flags or horizontal prisms are often used in aviation and military operations to visually signal the approach, caution, or boundaries of these areas. By drawing attention to hazardous terrain, these markers help ensure the safety of personnel and aircraft by providing necessary visual guidance in challenging environments. In contrast, the other types of terrain typically do not require such specific visual markers because their navigational or operational characteristics do not present the same level of risk.