The correct answer is D) Determining the amount of water (if any) in each cargo tank. "Thieving" a petroleum cargo refers to the process of measuring the amount of water (if any) in each cargo tank. This is an important step in accurately determining the net quantity of the petroleum cargo, as the presence of water in the tanks can impact the overall cargo volume and weight. Determining the water content is a standard practice to ensure the reported cargo figures are accurate and in compliance with regulations. The other answer choices are incorrect because they do not accurately describe the meaning of "thieving" a petroleum cargo. Option A) is about reducing gross cargo calculations to net amounts, which is a separate process from determining water content. Option B) is about adjusting cargo figures to match the draft, which is a different task. Option C) is about siphoning off cargo for shipboard use, which would be considered theft rather than a standard industry practice.