Which of the following is NOT a natural polymer?

We are asked to identify which of the given substances is NOT a natural polymer.

A natural polymer is a macromolecule that is produced by living organisms through biological processes. Let us examine each option:

Proteins (Option A) are natural polymers made up of amino acid monomers linked by peptide bonds. They are synthesised by all living organisms and serve structural, enzymatic, and regulatory roles.

Starch (Option B) is a natural polymer of glucose units linked by $$\alpha$$-glycosidic bonds. It is produced by plants as an energy storage molecule.

Rubber (Option C) — natural rubber is a polymer of isoprene (2-methyl-1,3-butadiene) obtained from the latex of the rubber tree (Hevea brasiliensis). It is therefore a natural polymer.

Rayon (Option D) is a semi-synthetic fibre. Although it is derived from natural cellulose (wood pulp), it undergoes extensive chemical processing — the cellulose is dissolved in alkali and carbon disulphide to form cellulose xanthate, which is then regenerated as fibre. Because of this chemical transformation, rayon is classified as an artificial or man-made polymer, not a natural one.

Hence, the correct answer is Option D.