Which of the following natural resources are not harmful to the environment?

Fossil fuels such as natural gas, coal, and petroleum are formed from the remains of fossils. It comprises carbon-based compounds. The burning of fossil fuels releases dangerous greenhouse gas which is carbon dioxide which is a major cause of global warming. It also leads to air pollution. Hence, the burning of fuels is a major cause of global warming.

But there are certain resources that give energy without causing any harm to the environment.

Thus, the given natural resources are classified as:

Harmful to the environment: Coal, petrol, wood ( they all produce harmful gases on their burning)

Unharmful to the environment: Hydroelectric energy, CNG, and wind energy. CNG is also a fossil fuel but it is much cleaner than other fossil fuels.