Zone refining is based on the principle of :

Zone refining is based on the principle of fractional crystallization.

As impurities prefer to remain in the melt, zone refining is based on fractional crystallization because only pure metals solidify on the melt's surface during solidification. The impurities in the metal are more soluble in the molten state during the zone refining process than they are in the solid state.

Hence, option B is correct.