Concept:Cross-linked polymers have a three-dimensional network structure formed by covalent bonds between different polymer chains.
Explanation:High density polythene (
HDPE) is a linear polymer with strong intermolecular forces, but it has no cross-links between its chains.
Low density polythene (
LDPE) has a branched structure, yet branching does not mean cross-linking.
Melamine reacts with formaldehyde to form melamine-formaldehyde resin, which is a thermosetting plastic with an extensive cross-linked network.
PVC (polyvinyl chloride) is a linear thermoplastic polymer and is not cross-linked in its usual form.
Therefore, among the given options, only melamine is a cross-linked polymer.
Answer:Option C — Melamine