ANS:D - none of these

Crystallisation of polymers can indeed be undesirable in certain applications, as it can lead to brittleness and reduced mechanical properties. In the case of celluloid, which is a type of polymer, crystallisation can indeed be prevented by adding certain substances. In the case of celluloid, crystallisation is prevented by adding plasticisers such as camphor. Plasticisers are substances that are added to polymers to improve their flexibility, workability, and resilience. Camphor acts as a plasticiser in celluloid, preventing the polymer chains from forming crystalline structures and thereby maintaining the desired properties of the material. Therefore, the correct answer is "camphor."