The correct answer is D) Location of the potable water spigots above the rim of the galley sink. This is the correct answer because having the potable water spigots located above the rim of the galley sink prevents the possibility of contamination from a backup of sewage or sanitary drains. By positioning the water supply above the sink, there is no risk of the water flowing back into the potable water system, even if there is a drain clog or backflow in the sink's drain line. The other options are incorrect because they do not effectively prevent contamination. Check valves can fail, a "P" trap only prevents backflow from the drain line itself, and locating the spigots below the sink rim still leaves the potential for backflow into the potable water system.