Which one of the following is an example of thermosetting polymer?

First, we need to understand what a thermosetting polymer is. Thermosetting polymers are plastics that, once set and hardened, cannot be reshaped by heating. This happens because they form strong cross-links between their polymer chains during the curing process, creating a rigid, three-dimensional network that doesn't melt when reheated. In contrast, thermoplastic polymers soften when heated and harden when cooled, allowing them to be remolded multiple times.

Now, let's examine each option:

Option A: Nylon-6,6. Nylon-6,6 is a synthetic polymer made from hexamethylenediamine and adipic acid. It is a polyamide and is classified as a thermoplastic. This means it can be melted and reshaped repeatedly when heated. Therefore, it is not a thermosetting polymer.

Option B: Bakelite. Bakelite is made from phenol and formaldehyde through a condensation reaction. During its formation, it undergoes extensive cross-linking, creating a rigid, infusible structure. Once cured, it cannot be softened or remolded by heating. This makes Bakelite a classic example of a thermosetting polymer.

Option C: Buna-N. Buna-N is a copolymer of butadiene and acrylonitrile. It is a synthetic rubber (elastomer) and is typically considered a thermoplastic elastomer. While it can be cross-linked (vulcanized) to improve properties, its base form is thermoplastic and can be reshaped by heating. Thus, it is not primarily classified as thermosetting.

Option D: Teflon. Teflon is polytetrafluoroethylene (PTFE), a polymer of tetrafluoroethylene. It has a high melting point but is a thermoplastic. It softens when heated and can be molded, meaning it is not thermosetting.

After evaluating all options, Bakelite (Option B) is the only one that fits the definition of a thermosetting polymer due to its irreversible cross-linked structure.

Hence, the correct answer is Option B.