If the plastics are hard, become soft and readily workable by the addition of certain compounds called:
Correct option is C. Plasticisers
A catalyst is a substance that can be added to a reaction to increasing the reaction rate without getting consumed in the process.
A telomere is a region of repetitive nucleotide sequences at each end of a chromosome.
Plasticizers are used to convert PVC, a rigid plastic, into a soft, flexible, and elastic material.
Vulcaniser is someone who vulcanizes rubber to improve its strength
So, if the plastics are hard, become soft and readily workable by the addition of certain compounds called plasticizers.
Hence, option $$C$$ is correct.