Which of the following energy sources does not require waste disposal?

a. geothermal

b. biomass

c. nuclear

d. coal

The answer is: A. geothermal. In this type of energy, the Earth's own heat is used as an energy source. No fuel is burned or otherwise used. Cold water is pumped down underground, then the Earth heats it up and hot water is extracted back. So there is no waste to dispose of.

On the other hand, biomass and coal are burned leaving ashes to dispose of. Nuclear fuel is depleted leaving radioactive waste.