Mandatory Signs are red with white legend. What do they indicate?

Answer: D — Red sign with white legend indicating an instruction that must be followed

Mandatory signs use a red background with white lettering to signal an instruction that must be followed. The red color communicates a command or prohibition, and the white legend tells you the exact action required, such as STOP or HOLD SHORT. When you see one of these signs, you are obligated to comply before you proceed, because failing to do so could create a dangerous situation on the airfield. This differs from warning signs (yellow with black lettering), which alert you to potential hazards, and from guidance signs (blue with white lettering), which provide information or directions rather than a required action.