Solution

On the basis of the mode of their formation or polymerisation, the polymers can be classified as addition polymers and condensation polymers.
Addition polymers are formed by reactions between monomer molecules having multiple bonds. Examples- polyethene.
Condensation polymers are formed by condensation between monomeric units with the elimination of small molecules such as water, ammonia, alcohol, etc. Examples- Nylon-6,6.
Hence, option $$D$$ is correct.