The correct answer is C) heating to allow water and sediment to settle out. Fuel oil is transferred to the settling tanks to allow water and sediment to separate from the fuel by gravity. This is an important step in the fuel oil purification process, as the presence of water and sediment in the fuel can cause issues with the proper atomization and combustion of the fuel in the engine or boiler. Heating the fuel in the settling tanks helps accelerate this separation process. The other answer choices are incorrect because they do not accurately describe the primary purpose of transferring fuel oil to the settling tanks. Option A is incorrect as the fuel is heated to aid in separation, not for proper atomization. Option B is incorrect as purging air bubbles is not the main reason for using settling tanks. Option D is incorrect as removing volatile gases is not the primary purpose of the settling tanks.