Introduction:
Purification of solid impurities is an important process that involves the removal of unwanted substances from a solid material. Various techniques are used for this purpose, such as distillation, sublimation, and crystallization. However, one of these techniques is not used for the purification of solid impurities.

Explanation:
Let's discuss each option and its relevance to the purification of solid impurities:

a) Distillation:
Distillation is a technique used to separate and purify liquid mixtures based on their boiling points. This process involves heating the mixture to vaporize the more volatile liquid component, which is then condensed and collected. Distillation is not typically used for the purification of solid impurities, as it primarily focuses on liquid mixtures.

b) Sublimation:
Sublimation is the process in which a solid directly converts into a gas without passing through the liquid state. This technique is commonly used for the purification of solid impurities. When a solid substance undergoes sublimation, the impurities present in the solid are left behind, resulting in a purified product.

c) Crystallization:
Crystallization is a technique used to purify solid substances by forming crystals from a saturated solution. The impurities present in the solution are often excluded from the crystal lattice, leading to a purified solid product. Crystallization is widely used for the purification of solid impurities, making it an essential technique in many industries.

d) None of these:
The option "None of these" implies that none of the mentioned techniques (distillation, sublimation, crystallization) are used for the purification of solid impurities. However, this statement is incorrect. Both sublimation and crystallization are commonly employed for the purification of solid impurities, whereas distillation is primarily used for liquid mixtures.

Conclusion:
The correct answer is option 'D' - None of these. This option is incorrect because sublimation and crystallization are indeed used for the purification of solid impurities. Distillation, on the other hand, is not typically employed for this purpose as it is primarily suited for the purification of liquid mixtures.