What is an ideal fuel?
A.It has calorific value
B.It is cheap
C.It is readily combustible
D.All of the above

Hint:The fuel which does not emit harmful gases on burning is known as ideal fuel. It is quite cost effective and easy to store and transport. It easily catches fire and burns so it is quite combustible and also has high calorific value.

Complete answer:
Fuel basically converts chemical energy into heat energy and some other forms of energy on burning. Ideal fuel has very low cost and readily catches fire so it is combustible in nature. Calorific value signifies the amount of heat produced by unit volume of substance when it undergoes complete combustion under constant pressure conditions. Ideal fuel has high calorific value which means it produces a high amount of heat energy on combustion. So ideal fuel is basically cheap, combustible and has high calorific value. Therefore, all the options given above are correct. Natural gas and liquefied petroleum gas (LPG) are examples of ideal fuels.
Ideal fuel should not emit toxic gases on combustion and it should also have proper ignition temperature. Ignition temperature provides activation energy required for combustion.

So, the correct choice is (D).

Note:
Ideal fuel is available readily and it is quite efficient for storage and transportation purposes. Natural gas has quite high calorific value and since it is a gas so it mixes with oxygen easily and becomes more efficient for combustion. It is environmentally friendly as it mostly produces carbon-dioxide and water vapors on combustion. The content of carbon-dioxide produced is also quite low as compared to other fuels. So natural gas is known as the cleanest burning fuel. It is a gaseous fuel. Solid fuel includes wood, coal and liquid fuel includes LPG, diesel, etc.