Which market structure is defined as having only one firm operating in the industry?

A market structure characterized by a single firm operating in the industry is known as a monopoly. In a monopoly, the sole producer has significant control over the market, which allows them to set prices and output levels without direct competition. This setup often arises in industries where the barriers to entry are extremely high, such as in cases involving significant capital costs, patents, or government regulations that restrict competition.

The lack of competition means that the monopolist can maximize profits by restricting supply to raise prices, which can lead to market inefficiencies and a decrease in consumer surplus. A monopoly has the ability to dictate terms in the market, which is distinctly different from other structures, such as oligopolies or duopolies where there are multiple firms involved, and therefore some level of competition exists.

In summary, the defining feature of a monopoly is its singular firm dominance in the market, fully explaining why this answer is the correct choice.