A reverberatory furnace keeps the burning fuel in a separate chamber from the metal charge and heats the charge mainly by radiant heat bouncing off the low, curved refractory roof above it, along with hot gases sweeping over the top of the charge. Because the metal never touches burning solid fuel directly, it stays free from impurities such as sulfur that the fuel would otherwise introduce, which matters a great deal for non-ferrous metals whose properties are sensitive to such contamination. This makes the furnace well suited for melting and refining copper and aluminium, both in primary smelting operations and in scrap-melting foundries. Pig iron instead comes from a blast furnace, and steel refining is carried out in a basic oxygen furnace or electric arc furnace, so those uses do not fit a reverberatory furnace. So the correct choice is option (D).