Combustion of most fuels releases $CO _2$ causing global warming. Global warming is the rise in the temperature of the atmosphere of the earth. 

A fuel is any material that can be made to react so that it releases chemical or nuclear energy as heat or to be used for work. The heat energy released by reactions of fuels is converted into mechanical energy via a heat engine.

An ideal fuel is cheap, readily available, easy to transport and readily combustible. It has high calorific value. It does not produce gases or residues that pollute environment.

Ideal fuel should have a proper ignition temperature, so that it can be burned easily. The ignition temperature of an ideal fuel should neither be too low nor too high. If the ignition temperature of the fuel is very low, then the fuel will catch fire too easily and hence it will be very unsafe to use it. On the other hand, if the ignition temperature is too high, then it will be very difficult to light the fuel.

A good fuel should have high calorific value, should be cheap, should not cause any pollution and should be easy to transport.

 the major components of all kerosenes are branched and straight chain alkanes and naphthenes (cycloalkanes), which normally account for at least $70\%$ by volume

The minimum temperature at which a substance starts to burn is called its ignition temperature. Different fuels have different ignition temperatures.  Low ignition temperature means highly inflammable. For combustion, ignition temperature should at least be achieved. Substance starts burning only at a temperature equal to ignition temperature or more than it.

Fuel efficieny can be measured as the amount of heat that is required by 1kg of fuel during combustion.  It is known as calorific value of the fuel.  The unit of measuring fuel efficieny is kJ/Kg.

While cooking, if the bottom of the vessel is getting blackened on the outside, it means that the fuel is not burning completely. When the fuel burns completely giving a blue flame, the bottom of the vessel remains clean. 

The producer gas is a mixture of nitrogen and carbon monoxide. Two thirds by volume is nitrogen and one third by volume is carbon monoxide. Carbon monoxide is combustible gas in the mixture. Nitrogen is non-combustible. producer gas is not considered as good fuel because it contains Nitrogen which does not burn.

An ideal gas must possess high calorific value, must have a moderate rate of combustion and must be easily available. The calorific values of coal, petrol, LPG are quite less than an ideal gas. The molecules present in an ideal gas obeys the gas laws and follows the Newtons laws of motion.

Ideal fuel shows the following characteristics:

• It should be  less expensive,
• It should be easily  available,
• It should produce a large amount of heat,
• It can be  transported  easily
• It has high calorific value.
• It should  burns easily in air at a moderate rate.

Hence option D is correct.

C.N.G (compressed natural gas) is a cleaner fuel because it causes less pollution by emitting fewer pollutants like carbon dioxide, carbon monoxide, sulphur dioxide, nitrogen dioxide, particulate matter, unburned hydrocarbons etc.

Ideal fuel has high calorific value. This ensures that more amount of heat energy is released from the combustion of unit mass of fuel. In other words, a fuel with high calorific value is better than a fuel having low calorific value.

LPG is a better domestic fuel than wood because of several reasons. Unlike wood, LPG burns without smoke. This makes the life of housewives more comfortable and they do not have to worry about blackening of pots and pans. Moreover, use of LPG as domestic fuel also rules out the chances of getting respiratory disorders which may happen when someone uses wood as kitchen fuel. Storage and transportation of LPG is easier compared to that of wood.

Solid fuel refers to various types of solid material that are used as fuel to produce energy and provide heating, usually released through combustion. Solid fuels include wood, charcoal, peat, coal, Hexamine fuel tablets, and pellets made from wood, corn, wheat, rye and other grains.

Compressed natural gas (CNG) (methane stored at high pressure) can be used in place of gasoline (petrol), Diesel fuel and propane/LPG. CNG is made by compressing natural gas (which is mainly composed of methane), to less than 1 percent of the volume it occupies at standard atmospheric pressure. 

Biogas can be considered as an ideal gas because it has an appropriate calorific value, easily processed from cow dung and other waste materials. It is environment friendly and burns smoothly without any residues. It contains negligible non-volatile materials.

Calorific value is the quantity of heat produced by the complete combustion of a given mass of a fuel, usually expressed in joules per kilogram.

Compressed natural gas (CNG) (methane stored at high pressure) can be used in place of gasoline (petrol), Diesel fuel and propane/LPG. CNG is made by compressing natural gas (which is mainly composed of methane, CH4), to less than 1 percent of the volume it occupies at standard atmospheric pressure. 

Smokeless fuels do not produce smoke when burned. Smokeless fuels are usually solid fossil fuels, such as: anthracite, coke, and charcoal.

A good fuel produces a huge amount of heat on burning, does not produce a lot of smoke, and is easily available.

Calorific value means the energy contained in a fuel or food, determined by measuring the heat produced by complete combustion of specific quantity, expressed as $J/kg$.