What is one characteristic of a monopoly?

A monopoly is defined by the presence of a single producer or seller in the market that controls the entire supply of a product or service. This characteristic allows the monopolist to dictate pricing and production levels since there are no competitors. The lack of competition means that consumers have no alternative options, and thus the monopolist can maintain market power.

In contrast, the other choices describe different market structures. Multiple firms producing similar goods relates to perfect competition or oligopoly, where competition exists among several firms. Close substitutes being available indicates a competitive market where consumers have options. Lastly, an environment with highly encouraged entry into the market signifies a competitive or open market structure, which opposes the nature of a monopoly, where entry is often restricted to maintain market power.