Which priority involves preparing positions for defenders to establish a secure area?

The priority that involves preparing positions for defenders to establish a secure area is entrenchment. This concept is focused on creating defensive positions that enhance the security and effectiveness of troops in a given area. Entrenchment techniques involve digging trenches, creating bunkers, or otherwise fortifying an area to make it more defensible against potential attacks. By establishing these secure positions, defenders are better equipped to protect themselves, maintain control over the territory, and respond effectively to threats.

Other options, such as supplementary positions, obstacles, and security, play important roles in overarching defense strategies but do not specifically refer to the act of preparing secure positions. Supplementary positions can refer to alternative locations where troops can regroup or reposition, while obstacles generally pertain to physical barriers designed to hinder the movement of adversaries. Security encompasses a broader range of protective measures but does not specifically highlight the physical preparation of defensive positions the way entrenchment does.