The correct answer is B) No fog signal is required. According to the Inland and International Navigation Rules, an unmanned barge being towed astern is not required to sound any fog signals. The responsibility for the fog signal falls on the vessel that is doing the towing, which in this case is the third vessel in the tow. The towing vessel must sound the proper fog signals, but the unmanned barges themselves do not need to sound any signals. The other answer options are incorrect because they describe fog signals that would apply to the towing vessel, not the third unmanned barge in the tow.