Melamine

The correct answer is option 1. Melamine.

An example of a heavily cross-linked polymer is melamine.

Melamine is a polymer with a three-dimensional, heavily cross-linked structure.

Cross-linking involves the formation of covalent bonds between polymer chains, creating a network structure.

This cross-linked structure gives melamine its unique properties, including hardness, durability, and resistance to heat and chemicals.

In contrast, polythene, nylon, and polyester are examples of polymers with less cross-linking, and they generally have more flexible and linear structures.