Which geographical factor is most useful for producing hydroelectricity from river water?

Correct answer: Slope and energy of water flow

Correct answer: C. Hydroelectricity is produced when moving water turns turbines connected to generators. For this process, water must have usable energy. A river flowing down a slope has gravitational potential energy, and as the water moves, this becomes kinetic energy. A dam or natural drop can create a difference in height, called head, which makes the water flow strongly through turbines. Therefore, river gradient and the energy of flowing water are especially useful geographical conditions. Option A is wrong because flat, still water has little movement energy for turning turbines. Option B is wrong because salinity and deserts do not provide the required river flow. Option C correctly identifies slope and flowing-water energy. Option D is wrong because water colour does not determine its power-producing ability. Memory cue: hydropower needs height plus moving water, not colour or stillness.