The correct answer is D) one meter from the deck. The reasoning for this is that the oxygen content in the tank must be measured at a location one meter from the deck when preparing to conduct a crude oil wash. This is in accordance with the regulations set forth by the U.S. Coast Guard for tank entry and crude oil washing procedures. The other options are incorrect because: A) the vent riser is not the appropriate location for this measurement, B) immediately above the level of the oil may not provide an accurate representation of the overall oxygen content, and C) at the top of the tank would not be the standard position specified in the regulations.