The correct answer is 2) Greater solubility of the impurity in the molten state than in the solid state.
Zone refining is a purification method based on the principle that impurities are more soluble in the molten state of the material than in the solid state. By creating a molten zone and moving it through a solid metal rod, impurities are selectively dissolved in the molten zone and then moved to one end of the rod, resulting in a more purified metal.