The correct answer is C) Furnace. In the boiler system, the furnace is where chemical energy from the fuel is converted to thermal energy through the process of combustion. The fuel, whether it's oil, gas, or solid fuel like coal, is burned in the furnace, and the heat generated is then transferred to the water and steam within the boiler system. The other components listed, such as the superheater, steam drum, and economizer, are involved in the heat transfer and steam generation processes, but they do not directly convert chemical energy to thermal energy like the furnace does.