Which signal must you display at night on a docked tank barge to show that it is loading or discharging flammable liquid cargo?

Correct Answer: Option D — Red light.

33 CFR 155.810 mandates that vessels transferring oil or hazardous materials must display a red light at night. This signal serves as a universal maritime warning to other vessels and personnel that dangerous cargo operations are in progress, ensuring compliance with safety protocols regarding the handling of flammable liquid cargoes.