Which from following is a thermosetting polymer?

Thermosetting polymers undergo irreversible cross-linking upon heating, forming rigid, infusible structures. Analyzing the options:

Buna-S is a synthetic rubber copolymer exhibiting thermoplastic behavior. Dacron (polyethylene terephthalate) and polyvinyls like PVC are also thermoplastic polymers.

Bakelite, a phenol-formaldehyde resin, forms extensive cross-links during curing and cannot be reshaped after polymerization, confirming its thermosetting nature.

The thermosetting polymer is therefore $\boxed{D}$.