Which of the following units is commonly used to measure the horsepower of a pump?

The commonly used unit to measure the horsepower of a pump is indeed horsepower. Horsepower is a unit that quantifies the power output of the pump and allows operators to understand the capacity and efficiency of the pumping system. This measure is crucial when selecting pumps for specific applications, as it helps to ensure that the pump can handle the required flow and head conditions effectively.

The other choices represent different measurement types that are not directly related to power output. Watts, for instance, is a unit of power as well but is a different measurement system (metric), which can be converted to horsepower, but is not itself used commonly for measuring pump horsepower. Gallons per minute is a flow rate measurement that indicates the volume of fluid being pumped, while feet usually refer to the head or height that a pump can lift fluid, which is another essential performance metric but does not quantify power directly.