The correct answer is A) the number of long tons of water displaced by a vessel afloat. Displacement refers to the weight of the water displaced by a vessel, which is equal to the weight of the vessel itself when it is afloat. This is in accordance with Archimedes' principle, which states that the buoyant force on a submerged object is equal to the weight of the fluid displaced by the object. The other options are incorrect because they refer to different naval architectural measurements: B) is the gross tonnage, C) is the deadweight capacity, and D) is the cubic capacity of the vessel, which are all separate and distinct from the displacement.