Concept:Orlon is a synthetic fibre obtained by addition polymerisation of a vinyl cyanide monomer.Explanation:Orlon is the common name for polyacrylonitrile.It is formed by the addition polymerisation of acrylonitrile.The structure of acrylonitrile is:CH2​=CH−CNRepeated units of acrylonitrile join together to give:[−CH2​−CH(CN)−]n​This polymer is known as orlon.Hence, the monomer used is acrylonitrile.Answer:Option B: acrylonitrile.