Concept:Thermoplastic materials are polymers that can be repeatedly softened by heat and hardened by cooling.
Explanation:Thermoplastics do not form permanent cross-links between their polymer chains.
When heated, the intermolecular forces between the chains weaken.
This weakening allows the material to become soft and flexible.
With further heating, the material eventually melts into a liquid.
On cooling, it hardens again and can be reshaped.
This process is reversible, so thermoplastics do not decompose on heating.
They also do not harden on heating; instead, they soften.
Thus, the correct behaviour is that they soften and melt when heated.
Answer:D. soften and melt on heating