The correct answer is B) National Geodetic Vertical Datum. On the Mississippi River, gage zero is the gage reading measured from the National Geodetic Vertical Datum (NGVD), which is a common reference plane used for all river gages and water level measurements. The NGVD is a standardized vertical datum established by the National Geodetic Survey, and it serves as the baseline for determining water depths and elevations along the river. The other options are incorrect because low water reference plane (A) is not the standard reference used for the Mississippi River, the lowest recorded river depth (C) can fluctuate and is not a fixed reference, and the highest recorded river depth (D) does not provide a consistent datum for all measurements.