Which of the following polymer can be formed by using the following monomer unit?

By using the given monomer unit of caprolactam formation of Nylon-6 takes place.

Nylon-6 can be prepared by ring-opening polymerisation of therefore option(iv) is the correct answer.

Nylon 6,6 can be prepared by polymerisation of Hexamethylene diamine and adipic acid,

Nylon2-nylon 6 is the alternating copolymer of glycine and amino caproic acid,

Melamine polymer is made from melamine and formaldehyde by condensation of two monomers. Therefore options (i),(ii),(iii) are incorrect.