Which form of defense uses a portion of terrain to mask the defensive position from enemy observation?

The reverse slope defense is designed specifically to utilize the terrain to provide cover and concealment for defensive positions from enemy observation. By positioning forces on the reverse slope of a hill or ridge, the defending unit is able to remain hidden from an attacking force, making it difficult for the enemy to accurately target them with fire or air support. This positioning not only helps to protect the defending unit from direct observation but also allows them to engage in surprise attacks against approaching enemy forces, enhancing their defensive capability.

In contrast, the other forms of defense have different focuses. A fortified defense emphasizes the use of constructed fortifications to enhance protective measures, while area defense focuses on controlling a designated region, often without the same emphasis on terrain masking. Mobile defense, on the other hand, is centered around the ability to shift locations and conduct counterattacks rather than relying on terrain concealment. Therefore, the reverse slope defense stands out because it specifically utilizes the terrain in a way that obscures defensive positions from enemy sight.