Which order is primarily used to alert units of an impending action?

The correct choice, which is the Warning Order, is utilized primarily to alert units about an impending action. A Warning Order serves as an early notification that prepares the units for an upcoming operation, allowing them to begin initial planning and preparations even before all details of the operation are finalized. This type of order is essential for maintaining operational tempo and readiness, as it ensures that units can mobilize efficiently and be prepared when the Operation Order or further instructions are issued.

In contrast, an Operation Order (often referred to as an OPORD) provides detailed guidance and orders for conducting an operation, including the specifics on the mission, execution, support, and command structure. Although crucial to the operation itself, it comes after the Warning Order and is not primarily meant for alerting units.

The 5 Paragraph Order represents a structured format for delivering an Operation Order, but again, it is not designed specifically for immediate alerts about impending actions. Finally, the Fragmentary Order (FRAGORD) is used to modify an existing Operation Order rather than to alert units to an entirely new action. It focuses on changes or updates to an already established plan. Thus, the function of the Warning Order clearly distinguishes it as the primary method for alerting units about imminent actions.