A water pump moves water at a rate of 15 gallons per minute. At this rate, how many gallons does the pump move in one hour?

Convert the rate from per minute to per hour. An hour has 60 minutes, so the pump moves $15 \times 60 = 900$ gallons in one hour, choice D. Dimensional analysis confirms the setup: $\frac{15 \text{ gal}}{1 \text{ min}} \times \frac{60 \text{ min}}{1 \text{ hr}} = 900 \frac{\text{gal}}{\text{hr}}$, with minutes canceling. Choice A (150) comes from multiplying by 10, and choice C (75) from multiplying by 5, neither of which matches the number of minutes in an hour. Choice B (60) is the number of minutes itself and ignores the pumping rate.