Following are the types of heat losses in boiler:

Heat loss due to heat carried by ashes

Heat loss due to dry flue gases

Heat loss due to surface radiation

Heat loss due to moisture present in fuel and steam formed by combustion of hydrogen in fuel.

Most heat losses are due to the high temperature of dry flue gases leaving the boiler. Boiler maintenance should be done regularly and be removing the scale deposited on the heat transfer tubes should be given prime importance as this scale prevents the transfer of heat between flue gas and water.

Hence, the correct option is (D).