The quantity of heat liberated when the unit volume of fuel is burnt and the product is allowed to cool down to room temperature is known asa.High calorific valueb.Net calorific valuec.All of the aboved.Low calorific value

The quantity of heat liberated when the unit volume of fuel is burnt and the product is allowed to cool down to room temperature is known as d. Low calorific value.

Here's why:

a. High calorific value - This is the total heat energy available in the fuel. It includes the energy used to evaporate and superheat the water content in the combustion products.

b. Net calorific value - This is the heat energy available after the water vapor has condensed and the heat for vaporizing the water content has been subtracted.

c. All of the above - This option is incorrect because the definition provided does not apply to all the options.

d. Low calorific value - This is the heat energy available after the combustion products have been cooled down to room temperature. It is also known as the useful calorific value. This matches the definition provided in the question.