The correct answer is D) preload. Preload refers to the temporary addition of sea water or other liquid into tanks to simulate the increased vertical loading that a vessel would experience from environmental forces such as waves and wind. This allows the vessel's structure to be tested under these simulated conditions before the vessel is launched and put into service. The other answer choices are incorrect because they do not accurately describe this specific testing procedure. Fixed load and basic load refer to other types of vessel loading, while liquid variable load is a more general term that does not specifically describe the temporary addition of liquid to simulate environmental forces.