The correct answer is A) A single-deck vessel may not be assigned dual tonnages. This is correct because the U.S. Coast Guard regulations state that a single-deck vessel must be assigned a single tonnage, either gross or displacement, and cannot be assigned dual tonnages. The dual-tonnage concept is typically applied to vessels with multiple decks, where the gross tonnage is measured for one purpose (e.g., documentation) and the displacement tonnage is measured for another purpose (e.g., load line). The other options are incorrect because B) refers to the incorrect tonnages, C) is not a regulatory requirement, and D) is not accurate, as the dual-tonnage mark placement is not specified in the regulations.