Which from following is a thermosetting polymer ?

Polymers are classified by thermal behavior into thermoplastics and thermosetting polymers. Thermoplastics soften when heated and can be remolded, as seen in polyvinyl chloride (PVC), polystyrene, and nylon 6,6. Thermosetting polymers undergo irreversible hardening upon heating and cannot be remolded; examples include urea-formaldehyde resin, bakelite, and melamine-formaldehyde resin.

Among the options: $A$, $B$, and $D$ are thermoplastics, while $C$ (urea-formaldehyde) is thermosetting.

The thermosetting polymer is $\boxed{C}$.