The correct answer is A) port and starboard sides. Load line markings, which indicate the maximum draft at which a vessel can safely operate, are required to be displayed on both the port and starboard sides of the vessel. This is specified in the International Convention on Load Lines, which sets the international standards for load line requirements. The load line markings must be clearly visible from both sides of the vessel to ensure compliance and safety. The other answer choices are incorrect because the load line markings are not located solely on the port side (B), starboard side (C), or stern (D) of the vessel. They must be displayed on both the port and starboard sides as per the regulations.