What is defined as the rate at which work is done or energy is gained or expended?

The correct answer is power because it specifically measures the rate at which work is performed or energy is transferred over time. In physics, power quantifies how quickly energy is being used or generated, expressed in units such as watts. This concept is critical in various applications, from electrical systems to mechanical devices, where understanding how quickly energy is being converted into work helps evaluate efficiency and performance.

For instance, in the context of engines or electrical devices, a higher power rating signifies that the device can do more work in less time, enhancing overall functionality. This distinction sets power apart from related concepts such as energy, which refers to the capacity to do work, and work itself, which is the measure of energy transfer that occurs when a force acts over a distance. Force, while crucial in determining the movement of an object, is not a measure of the rate at which energy is used, making power the most appropriate selection here.