A monopoly is a market where there is only one seller of the product and this seller sets the price alone. The Türk Telekom, for example, is a monopoly in internet infrastructure. All the other internet providers have to use Telekom’s infrastructure. Likewise, TCDDY is a state owned monopoly in railway transportation. There are also local monopolies. Many cities in Turkey are served by a single airline (typically THY subsidiary Anadolu Jet). In smaller towns, many goods and services could be offered by a single seller. There are a few worldwide monopolies as well: SpaceX, for example, is the only company selling touristic trips to the space. The correct answer is D.