Which of the following is a polyamide?

A polyamide is a polymer in which the repeating units are linked through the amide group $$\left(-CONH-\right)$$. The amide linkage is formed by the condensation of a carboxylic acid group with an amine group, releasing a molecule of water for every linkage created.

The common man-made polymers given in the options can be classified as follows:

Option A (Teflon): Polymer of $$CF_2=CF_2$$. The repeating unit is $$-CF_2-CF_2-$$. It contains only carbon-fluorine and carbon-carbon bonds, with no $$-CONH-$$ linkages. Hence it is a polyfluoro-alkene, not a polyamide.

Option B (Nylon-66): Prepared by the condensation of hexamethylene diamine $$\left(H_2N-(CH_2)_6-NH_2\right)$$ with adipic acid $$\left(HOOC-(CH_2)_4-COOH\right)$$. Each condensation step produces one $$-CONH-$$ (amide) linkage:
$$-NH-(CH_2)_6-NH-CO-(CH_2)_4-CO-$$. Because the backbone is held together entirely by amide bonds, Nylon-66 is the textbook example of a polyamide.

Option C (Terylene, Dacron): Obtained by condensation of ethylene glycol and terephthalic acid, giving the repeating ester linkage $$-COO-$$. Therefore it is a polyester, not a polyamide.

Option D (Bakelite): Synthesised from phenol and formaldehyde; the network contains $$-CH_2-$$ and $$-O-$$ bridges, but no amide link. Hence it is a phenol-formaldehyde resin, not a polyamide.

Only Nylon-66 contains the characteristic $$-CONH-$$ linkages throughout the chain, so it is the only polyamide among the choices.

Option B which is: Nylon-66