What is the principle of zone refining of metals?
(A) Greater mobility of the pure metal than that of the impurity.
(B) Higher melting point of the impurity than that of the pure metal.
(C) Greater noble character of the solid metal than that of impurity.
(D) Greater solubility of the impurity in the molten state than in the liquid.

Hint: Zone refining is based on the solidification of pure metal by concentrating the impurities at one surface in the melting state. The zone refining is based on the advanced separation techniques.

Complete step by step answer:
* First, let us know about the zone refining of metals. It is considered to be an important melting technique, also known as travelling melting technique, or floating zone process.
* From the name we can say it is a method of purifying the metals like silicon and germanium. This technique is centred on the fractional distillation technique.
* In this process, a solid is refined by passing from a number of molten zones in one direction.
* So, impurities are concentrated in the melting state, and pure metal solidifies on the surface.
* Thus, the ratio of impurity present in the solid state to the impurity in the liquid or melting state defined as the segregation coefficient is generally less than 1.
* Now, we can conclude that the impurities present are more soluble in the melting state.
Therefore, the principle of zone refining of metals is greater solubility of impurity in the molten state than in the liquid, or solid state.
The correct option is D.

Note: Keep in mind the separation techniques, and presence of an ore of metal in the different physical states i.e. melt, liquid, or solid. The segregation coefficient value should be remembered.