The two compounds shown react together to give a polymer used as a fibre. Which description of the polymerisation is correct?

Hexane-1,6-diamine and hexanedioic (adipic) acid have two reactive groups each. They link through amide bonds with elimination of water, so growth is step-growth and the product is nylon 6,6, made of two different monomers. A chain-growth radical route needs C=C bonds, which neither compound has, and no HCl is involved because no acid chloride is used.