Which fuel has the highest calorific value?
(A) Hydrogen gas
(B) Methane
(C) Wood
(D) Petrol

Hint:The term which is used to measure the energy that is produced by the process of combustion is calorific value. The calorific value varies for the different types of the fuel that offers combustions, it may include diesel, petrol, kerosene, coal etc.

Complete step by step solution:
The heat energy is derived from the sources like wood, charcoal, petrol, kerosene, methane etc. Hence these heat sources are also called fuels. The term calorific value is defined as the amount of heat required for the complete combustion of the fuel of the quantity of one kilogram. The hydrogen is the colorless and odorless gas and the calorific value of the hydrogen gas is $150000\,KJ/Kg$. The methane has one carbon and four hydrogen atoms in it and the calorific value of the methane is $50000\,KJ/Kg$ . The dry wood is also used commonly but not more effectively than the other fuels but it is easily available. The calorific value of the wood is $17000$ to $22000\,KJ/Kg$ . Petroleum is the fossil fuel that is formed due to the dead plants and animals under the earth surface. The calorific value of the petrol is $45000\,KJ/Kg$ . By comparing all these options, the hydrogen gas possesses the greatest calorific value and wood has the lower calorific value.

Thus the option (A) is correct.

Note:The unit of the calorific value is $jouleK{g^{ - 1}}$ . The other examples of the fuel are cow dung, coal, diesel, compressed natural gas, liquefied petroleum gas, bio gas etc. Out of which, coal, petroleum and natural gas are the major fossil fuels.