A short fragment of the polymer chain (two repeat units, end groups shown) is drawn. Which of the structures shown is the monomer of this addition polymer?

A cyano group on alternate backbone carbons identifies polyacrylonitrile (orlon, acrilan). Its monomer is acrylonitrile, propenenitrile, $\mathrm{CH}{}_2=\mathrm{CH}-\mathrm{CN}$. Acrylamide would give an amide side group, fumaronitrile would put a cyano group on every backbone carbon, and acetonitrile has no C=C.