The correct answer is C) To heat the fuel enough for proper atomization. Recirculating the fuel oil before lighting off a cold boiler allows the fuel to be heated gradually, which is necessary for proper atomization. When the fuel is cold, it has a higher viscosity and requires more heat to achieve the proper atomized spray pattern for efficient combustion. Recirculating the fuel allows it to be gradually warmed, ensuring that it will atomize correctly when injected into the boiler. The other options are incorrect because they do not directly address the need for proper fuel atomization. Option A is not the primary reason for recirculation, option B is about building up pressure, and option D is about water removal, which is a separate concern.