Concept:Catalogs are product lists sent by sellers who operate without physical shops.Explanation:Mail order firms mainly sell their goods from a distance.They cannot display stock in store windows or aisles.So they rely on catalogs to show product details, prices, and images.Customers read the catalog and order using a form, phone call, or the internet.Hence, the catalog is the primary selling tool for mail order firms.Answer:C. mail order firms