Step 1: Concept
Cross-linked polymers (network polymers) are formed by polyfunctional monomers and contain covalent bonds between various linear polymer chains.
Step 2: Analysis
- HDPE, LDPE, and PVC are linear or branched-chain polymers.
- Melamine-formaldehyde is a classic example of a cross-linked thermosetting polymer.
Step 3: Conclusion
Hence, Melamine is the cross-linked polymer.
Final Answer: (C)