The correct answer is D) the watertight portion of a vessel above the waterline. Reserve buoyancy refers to the volume of the ship above the waterline that remains watertight and contributes to the vessel's overall buoyancy. This watertight volume is crucial for a ship's stability and survivability, as it provides additional buoyancy in the event of flooding or other incidents that could compromise the vessel's stability. The other answer options are incorrect. Option A refers to the void portion below the waterline, which is not the definition of reserve buoyancy. Option B, "also called GM," is incorrect as GM (metacentric height) is a different stability characteristic. Option C, about the number of transverse watertight bulkheads, is not directly related to the definition of reserve buoyancy.