Which option describes a device or method that uses less energy?

The term "energy-efficient" specifically refers to devices or methods designed to use the least amount of energy necessary to perform a given function. This efficiency often means that the device can operate at a reduced energy consumption level while still delivering the same level of performance and output. For example, energy-efficient appliances consume less electricity compared to standard models, which translates into lower energy costs and reduced environmental impact.

The other options, while related to energy usage, do not directly capture the aspect of efficiency. Renewable energy refers to sources that can be replenished naturally, such as solar or wind power, but does not necessarily indicate how efficiently that energy is used. Similarly, "energy-saver" and "power-efficient" imply reductions in energy usage or power consumption, but they are not as specific as "energy-efficient" in defining the relationship between energy input and task output. Thus, "energy-efficient" provides the clearest and most precise definition of a method or device that optimally reduces energy consumption.