When taking the road test for a military license, what must the vehicle display at all times?

During a road test for a military license, it's essential for the vehicle to display a sign that reads "STUDENT DRIVER." This requirement is in place to ensure that other drivers on the road are aware that the vehicle is being operated by an individual who is in the process of being trained. This awareness promotes safety, as other drivers can exercise caution when approaching or interacting with the vehicle.

In a military context, where precision and safety are paramount, the "STUDENT DRIVER" sign alerts others to the fact that the maneuvering may not yet reflect the standard of an experienced driver. This can help prevent misunderstandings and minimize potential hazards on the road.

The other options, while they may have their own importance in different contexts, do not serve the same purpose as the "STUDENT DRIVER" sign during a road test. A military insignia, warning flag, or registration sticker does not convey that the vehicle is being operated by a learner and does not provide the same level of necessary caution to surrounding drivers. Hence, the correct answer is focused on fostering a safe learning environment on the road.