ANS:D - both (a) and (c)

The caking of crystals can be prevented by maintaining low critical humidity and coating the product with inert material. Caking of crystals occurs when moisture is absorbed by the crystals, causing them to agglomerate and form lumps or cakes. By maintaining a low critical humidity, the surrounding environment is kept relatively dry, reducing the likelihood of moisture absorption by the crystals and thus preventing caking. Additionally, coating the crystals with an inert material creates a barrier that limits the exposure of the crystals to moisture in the environment. This coating helps to prevent moisture from reaching the crystal surfaces, thereby reducing the risk of caking. So, the correct option is both (b) and (c).