The zone refining process of metals is based on the principle of: excess noble character of the liquid metal than that of impurity; lower melting point of the impurity than that of pure metal; greater solubility of impure metal than that of impurity; greater solubility of the impurities in the molten state than in the solid.

Zone refining is particularly used when metals are required in high degree of purity. It is based on the principle that when an impure metal in a molten state is allowed to cool, only the metal crystallizes while the impurities remain present in the molten state (mass) or melt. So the zone refining process of metals is based on the principle of Greater solubility of the impurities in the molten state than in the solid. Hence option D is correct.