Which one of the following substance is NOT synthetic?

The correct answer is A. Fibroin is the structural protein that forms the core of raw silk, secreted by the silkworm (Bombyx mori) and hardened into the fibre we call silk. It is a natural polymer of amino acids, mainly glycine, alanine and serine, arranged in beta-pleated sheets.

The other three are man-made polymers:

• Lexan is a GE trade name for polycarbonate, a tough, transparent thermoplastic used in bullet-resistant glazing, CDs and helmets.
• Neoprene is polychloroprene, a synthetic rubber first produced by DuPont in 1930, used in wetsuits, gaskets and conveyor belts.
• Teflon is the DuPont trade name for PTFE (polytetrafluoroethylene), discovered accidentally in 1938 and used for non-stick coatings and chemical-resistant seals.

The concept here is the difference between natural and synthetic polymers. Natural polymers such as proteins, cellulose and natural rubber come from living organisms, while synthetic polymers are made through industrial polymerisation. This links to the Science and Technology section and to Mains GS-III on new materials and their environmental impact, especially the persistence of fluorinated compounds like PFAS.

Trap: Students pick Lexan or Neoprene because they recognise them as brand names and assume trade names are natural. Brand-name recall does not tell you the chemistry; ask whether the substance is manufactured in a factory or secreted by an organism.