Some polymers are elastic because they are made of
a. carbon.
c. sugar and alcohol.
b. phosphate ladders.
d. cross-linked chains.

Option (d) is the correct answer.

Explanation:

A polymer is defined as a substance which contains repeating units of monomers. Basically, a polymer is a long chain of monomer units as 'poly' means many and "mer" means units.

Hence, a polymer contains many units. Some polymers are long linear chains and some are cross-linked polymers.

A polymer is elastic in nature because it contains chains which are cross-linked. These chains have the ability to adjust themselves according to the force or stress applied on them.

As this cross-linkage is covalent in nature therefore, it makes sure that when stress is removed then the elastomer will return to its original shape.

Thus, we can conclude that some polymers are elastic because they are made of cross-linked chains.