which of the following is most easily economically refined by fractional distillation

Answer: A) Zn

Distillation is used to refine metals with low boiling points, such as zinc, cadmium and mercury. Zinc boils at about 907 °C, so the impure metal can be heated until zinc vaporises; the vapour is condensed to give pure zinc, leaving the non-volatile impurities behind (fractional distillation also separates it from cadmium and lead).

Nickel, copper and iron have very high boiling points (well above 2500 °C), so distillation is not an economical way to refine them. Copper is refined electrolytically, and nickel by the Mond process.