Identify cross linked polymer from following.

The question requires identifying which polymer has a cross-linked structure from the given options.

High density polythene (HDPE) is a linear polymer with high crystallinity and thermoplastic behavior, lacking cross-links.

Low density polythene (LDPE) is highly branched but remains thermoplastic without cross-linking.

Melamine refers to melamine-formaldehyde resin, a thermosetting polymer characterized by extensive cross-linking that forms a rigid, three-dimensional network structure, rendering it infusible upon heating.

PVC (Polyvinyl chloride) is a linear thermoplastic polymer without cross-links.

Melamine is the cross-linked polymer among the options.