Which one of the following fuels is considered environment friendly?
A. Coal
B. Petroleum
C. Natural gas
D. Firewood

Hint: Any material which can react with other substances so it can release energy as thermal energy which can be used for work. Earlier fuels were described as any material which has the capacity to release chemical energy.

Complete answer:
Earlier fuels were described as any material which has the capacity to release the chemical energy which can be used for work. Now fuels are described as any material which can react with any other substance so it can release energy in thermal energy.

Option A is incorrect because it is a combustible brownish black sedimentary rock substance which is formed as rock strata. They mostly consist of carbon and variable amounts of other substances like nitrogen, oxygen and sulfur. It takes millions of years for the formation of coal by the dead remains of plants and animals by the heat and pressure of deep burial. Coal mining and coal fueling causes major damage to human life and to the environment.

Option B is incorrect because petroleum, also known as crude oil, is yellowish black liquid found beneath the surface of earth. Components of petrol are separated by the process known as fractional distillation. It contains naturally occurring hydrocarbons with some organic compounds. Petrol is recovered mostly by oil drilling.

Option C is correct because natural gas is extracted from the petroleum beneath the surface of earth. It occurs just above the crude oil and is lighter than petrol. This is also formed by the same process petroleum is formed. Because of high temperature and pressure, the dead remains of plants and animals are converted into natural gas. It is less harmful as compared to coal and petroleum.

Option D is incorrect because any wooden material which is used as fuel is known as firewood. It is not highly processed and it can be dry or fresh. It is a renewable resource, and the demand for this fuel has an ability to regenerate on local or at regional level.

Hence, option C is the correct answer.

Note:
Earlier fuels were described as any material which has the capacity to release the chemical energy which can be used for work. Natural gas is considered as an environment friendly fuel because it does not produce any harmful gas while combustion. Other three produce harmful gases like carbon dioxide and sulfur dioxide which cause harm to animals and the environment.