The correct answer is B) Net tonnage is the gross tonnage less certain deductions for machinery and other areas. Gross tonnage refers to the total enclosed volume of a ship, while net tonnage is the gross tonnage minus certain deductions for things like crew spaces, machinery spaces, and other areas not used for cargo. The net tonnage represents the actual usable space on the ship, whereas the gross tonnage is a measure of the entire enclosed volume. Net tonnage is the more relevant figure for determining a ship's cargo capacity and other practical purposes. The other answer choices are incorrect - A) refers to a different concept, C) is an inaccurate description of net tonnage, and D) is false as there is a clear difference between the two measurements.