When carrying hazardous materials, you must:

Answer: D — Have a red flag on the back of the vehicle

When carrying hazardous materials, you must have a red flag on the back of the vehicle, as this is a required marking for any vehicle transporting hazardous materials. Options A and B are basic safety measures and should always be followed while driving, but they are not specific to transporting hazardous materials. Option C may be used by emergency response vehicles, but it is not required for vehicles carrying hazardous materials. Therefore, the best answer is D.