The correct answer is A) heating to allow water and sediment to settle out. Fuel oil is transferred to settling tanks to allow water and sediment present in the fuel to separate and settle out before the fuel is used. This process helps ensure that only clean, contaminant-free fuel is supplied to the engine, which is important for efficient and reliable engine operation. The other answer choices are incorrect because B) refers to heating the fuel for proper atomization, C) refers to removing volatile gases, and D) refers to purging air bubbles, none of which are the primary purpose of transferring fuel to settling tanks.