The correct answer is A) 215-foot naval auxiliary vessel. Vessels required to have a Pollution Placard posted on board are commercial vessels 26 feet or longer. This includes recreational vessels used for commercial purposes, such as a 50-foot cabin cruiser used for charter. However, naval auxiliary vessels like the 215-foot vessel in option A are exempt from this requirement, as they are considered public vessels not engaged in commercial service. The other options, B) 75-foot towing vessel and D) 150-foot unmanned tank barge, would both be required to have a Pollution Placard, as they are commercial vessels over 26 feet in length. Option C) 50-foot cabin cruiser used for pleasure only would also need a Pollution Placard, as it is a commercial vessel even though used for recreational purposes.