Which type of heat sourc is the cleanest

I'm not sure if this is the correct answer you are looking for, but i believe that boiling water produces the cleanest heat source.

Solar is the cleanest because we do not have to do anything to get to it. but I believe in terms of environmental impact geothermal heat is a great source of heat. I'm not sure what you mean by cleanest but I would say either solar or geothermal energy.