Final answer:
The residential per capita water demand for the United States is approximately 120 gallons per person per day, which is most closely represented by option C in the provided choices.
Step-by-step explanation:
The residential per capita water demand for the United States can be understood through the average water use in American households. When considering indoor water use, it averages to approximately 125 gallons per household per day (gphd), or when looking at usage based on the number of people in a household, it is about 100 gallons per person per day. Outdoor residential water use varies by location and can average to 138 gallons per household per day. However, the previously mentioned indoor usage is the more typical reflection of residential water usage per person, making the closest answer to the student's question option C) 120 gal/capita/day.