The correct answer is C) A vessel with wooden planks on a steel deck will have the deck line marked at the intersection of the upper line of the wood sheathing with the side shell. This is correct because the deck line is defined as the intersection of the freeboard deck (the uppermost complete deck) with the side of the vessel. On a vessel with a wooden deck, the deck line is marked at the upper edge of the wood planking, where it meets the side shell, rather than at the underlying steel deck. The other options are incorrect because they do not accurately describe how the deck line is defined for a vessel with a wooden deck. Options A, B, and D refer to situations with different deck configurations, such as a rounded stringer-sheer plate or a freeboard deck without wood sheathing.