Which market structure is characterized by a single seller who controls the entire market and sets prices to maximize profits?

The market structure you are asking about is called a monopoly. In a monopoly, there is only one seller in the market, which means that this seller has complete control over the supply of a product or service. Because there are no competitors, the monopolist can set prices at a level that maximizes their profits without worrying about losing customers to other businesses. For example, if a company is the only provider of a specific medication, it can charge a higher price since consumers have no alternative options. This ability to control prices and supply is what defines a monopoly and allows it to maximize profits.