The monomer of natural rubber is

A monomer is the small molecule that undergoes repeated polymerization (addition or condensation) to build up a large polymer chain; the repeating unit inside that chain is derived from the monomer's own structure. Natural rubber is a naturally occurring polymer, so its monomer is the single small molecule from which that chain is built, unit by unit.

Natural rubber, obtained from the latex of the rubber tree (Hevea brasiliensis), is chemically cis-1,4-polyisoprene. It is built when the diene monomer isoprene (2-methylbuta-1,3-diene) undergoes repeated 1,4-addition polymerization; one double bond is retained per repeat unit, and the resulting cis arrangement of these units is what gives natural rubber its elasticity.

• Terylene names a polyester fibre made by condensing ethylene glycol with terephthalic acid — a manufactured textile polymer, not a diene monomer, so it cannot be the monomer of natural rubber.

• Orlon is the trade name for the acrylic fibre polyacrylonitrile, built from the monomer acrylonitrile — again not the diene monomer that forms natural rubber.

• Neoprene is itself a synthetic rubber, a polymer of the monomer chloroprene — a finished polymer, not a small monomer molecule, so it cannot be the answer either.

The systematic chemical name for natural rubber, poly-isoprene, itself names isoprene as the monomer from which it is built.