The correct answer is D) volume of intact space above the waterline. Reserve buoyancy refers to the volume of intact space above the waterline that can provide additional buoyancy in the event of flooding or damage to the vessel. This extra buoyancy helps keep the vessel afloat and stable, even if some compartments become flooded. The other answer choices are incorrect because they do not accurately describe the definition of reserve buoyancy. Option A refers to buoyancy differences in salt and fresh water, option B describes the relationship between buoyant and gravity forces, and option C refers to unoccupied space below the waterline, which is not the definition of reserve buoyancy.