A(n) __________ is a preliminary notice of an order or action. Matrix order Operation order Warning order Fragmentary order

The correct choice for the blank is "Warning order."

A Warning Order (WARNO) is a preliminary notice informing personnel of an impending order or action. It is generally used in military operations to prepare troops or support services for an upcoming mission or task. This kind of order provides essential information to allow units to start planning and to allocate resources accordingly before the final order is issued. A Warning Order may include details about the mission, the timeline, and any immediate actions that need to be taken. This preparation enhances operational readiness and ensures all personnel are aware of what to expect, ultimately improving the efficiency of the overall mission.

So, the complete sentence would read: "A Warning Order is a preliminary notice of an order or action."