What is a monopoly?

A monopoly is defined as a market structure where a single seller dominates the entire market for a particular good or service. This seller has significant control over the pricing and availability of the product because there are no close substitutes available for consumers. In a monopoly, the absence of competition allows the monopolist to set prices higher than would be possible in a competitive market, leading to significant market power.

In contrast, the other options describe different market structures. A market with many sellers indicates perfect competition, where numerous firms operate and no single entity can control prices. A market with no barriers to entry typically describes a competitive market where new firms can easily join the industry. Lastly, a market with multiple competing firms refers to an oligopoly or perfect competition, both of which entail multiple sellers, thereby diffusing market control among them. Thus, the defining characteristic of a monopoly is the presence of a single seller, justifying why this option is accurate.