Which of the following is a polyamide?

Polymers are classified according to the type of linkage that joins their repeating units (monomers).

• When the linkage is an ester group (-COO-), the polymer is called a polyester (e.g., terylene).
• When the linkage is an ether group (-O-), the polymer is called a polyether (e.g., polyethylene glycol).
• When the linkage is a sulphide group (-S-), the polymer is called a polysulphide (e.g., Thiokol).
• When the linkage is an amide group (-CONH-), the polymer is called a polyamide.

Nylon-6,6, nylon-6, and nylon-11 are well-known examples of polyamides; their repeating units are joined through the $$-CONH-$$ bond produced by condensation of carboxylic acid and amine functional groups.

Examining the given options:
Option A — Teflon (polytetrafluoroethylene) contains $$-CF_2-CF_2-$$ units; no amide bond.
Option B — Orlon (polyacrylonitrile) contains $$-CH_2-CH(CN)-$$ units; no amide bond.
Option C — Nylon (e.g., nylon-6,6) contains repeating $$-NH-(CH_2)_6-NH-CO-(CH_2)_4-CO-$$ units with the $$-CONH-$$ linkage. Hence it is a polyamide.
Option D — Terylene (polyethylene terephthalate) is a polyester; its linkage is $$-COO-$$, not $$-CONH-$$.

Therefore, the polymer that is a polyamide is:

Option C which is: Nylon