D. Gallons
Your first hint is to look at what unit the problem itself uses. If the problem refers to similar quantities with a unit, chances are, that unit is correct answer (unless it specifically says otherwise). However, if you want to be sure your answer is correct, ask yourself what the quantity is. Is the quantity a measurement of weight, distance, mass, or volume? Since we are dealing with pool water, it would be volume. The measurement of any liquid is typically volume (unless specified otherwise). Pounds and tons measures weight. While bushels can measure volume, it is more commonly used to measure mass or weight. Gallons is known universally for measuring volume, so your best bet would be to use gallons. Hope this helps!