The correct answer is B) Determining the amount of water (if any) in each cargo tank. The term "thieving" in the context of petroleum cargo refers to the process of measuring the amount of water, if any, that has accumulated in the bottom of each cargo tank. This is an important step in calculating the net amount of the petroleum cargo, as the water content needs to be subtracted from the gross cargo volume. Correctly determining the water content, or "thieving" the cargo, ensures an accurate accounting of the actual petroleum being transported. The other answer choices are incorrect because they do not accurately represent the meaning of "thieving" a petroleum cargo. Reducing the gross cargo calculations (A), adjusting the cargo figures to coincide with the draft (C), and siphoning off cargo for shipboard use (D) are not the correct definitions of "thieving" in this context.