The correct answer is A) Determining the amount of water (if any) in each cargo tank. The term "thieving" in the context of a petroleum cargo refers to the process of measuring the amount of water that may have accumulated in the cargo tanks. This is an important step to ensure accurate cargo calculations and compliance with regulations. Reducing the gross cargo calculations to net amounts (B), adjusting the cargo figures to coincide with the draft (C), or siphoning off cargo for shipboard use (D) are not the correct meanings of "thieving" a petroleum cargo.