Concept:A mail order firm sells goods by post using a catalogue without a physical retail shop.Explanation:Customers select products from a printed or online catalogue.Orders and payments are sent by post, and goods are delivered directly to customers.This business model avoids maintaining a chain or department store building.Among the options, chain store, department store, and shopping mall all operate from physical premises.Only the mail order firm matches the description of selling by post with a catalogue.Answer:C. mail order firm