The correct answer is D) Cycling the potable water pump on and off by the action of the potable water hydro-pneumatic tank pressure switch in response to system demand changes. This is the correct answer because on small commercial cargo vessels with limited crew and potable water usage, the potable water system pressure is typically maintained using a hydro-pneumatic tank. The pressure switch in the tank monitors the pressure and cycles the pump on and off to maintain the desired pressure range as water is consumed and the tank level drops. This is a common and effective way to manage potable water pressure on these types of vessels without the need for continuous pump operation. The other options are incorrect because they do not accurately describe the typical potable water pressure maintenance method used on small commercial cargo vessels. Option A would not maintain consistent pressure, option B would waste energy, and option C would not respond to changes in demand.