Concept:Thermosetting polymers undergo irreversible cross-linking on heating and cannot be remoulded.
Explanation:Urea-formaldehyde resin is a thermosetting polymer.
It is produced by the condensation polymerisation of urea and formaldehyde.
On heating, it forms a highly cross-linked, three-dimensional network structure.
Once set, it cannot be softened or reshaped by further heating.
Polystyrene and Terylene are thermoplastics, as they soften on heating and can be remoulded.
Buna-S is a synthetic elastomer, not a thermosetting polymer.
Answer:Hence, the correct option is A: Urea-formaldehyde resin.