What is the primary source of electricity generated in hydroelectric power systems?

The primary source of electricity generated in hydroelectric power systems is flowing water. Hydroelectric power harnesses the energy from moving water, usually from rivers or streams, to turn turbines connected to generators. As water flows over dams or through turbine systems, its kinetic energy is converted into mechanical energy and then into electrical energy. This method is considered renewable because it relies on the earth's water cycle and does not deplete natural resources or produce greenhouse gases during operation.

In contrast, other options represent different energy sources. Wind is used in wind turbines to generate electricity, sunlight is harnessed through solar panels, and geothermal heat comes from the earth’s internal heat to generate energy. While these are all valid renewable energy sources, they are not the mechanisms by which hydroelectric power systems generate electricity.