The correct answer is (c) Solar energy.
Explanation:
Solar energy is considered the cleanest energy source because it produces no harmful emissions when harnessed, as it directly utilizes the radiation from the sun.
Why other options are incorrect:
(a) Nuclear energy:
While considered clean in terms of emissions, nuclear energy poses risks due to potential accidents and the disposal of radioactive waste.
(b) Thermal energy:
Thermal energy often involves burning fossil fuels, which release greenhouse gases and other pollutants into the atmosphere.
(d) Geothermal energy:
Although relatively clean compared to fossil fuels, geothermal energy can still produce some emissions during the extraction process.