The correct answer is B) grain cubic. The grain cubic (or grain capacity) refers to the internal volume of a cargo hold measured from the inside of the side shell, the underside of the deck, and the tank top. This is the standard way of measuring the cargo space capacity of a vessel, as it represents the total available volume for storing cargo. The other options are incorrect: A) Deadweight space refers to the total weight-carrying capacity of a vessel, not its cargo volume. C) Measurement space is a broader term that can refer to various volumetric measurements of a vessel, not specifically the cargo hold volume. D) Gross tonnage is a measure of the overall size and volume of a vessel, not the specific cargo hold capacity.