Final answer:
A) Red license plates. Any vehicle operated by a GDL driver under 21 must display red license plates.
Step-by-step explanation:
Any vehicle operated by a GDL driver under 21 must display red license plates. This requirement is part of a graduated driver licensing system, which is designed to gradually introduce young drivers to the responsibilities of driving. The red license plates serve as a visual cue to other drivers, indicating that the driver is still in the learning stage and may have certain restrictions or limitations.