Which definition describes a monopoly?

A monopoly is defined as a market structure in which a single seller dominates the market and has substantial control over the price and supply of a good or service. This occurs because there are no close substitutes available for the product they offer, allowing the monopolist to influence the market significantly without competition. Monopolies can arise due to various factors, including unique resources, government regulation, or technological advantages that prevent other firms from entering the market.

In the context of the choices provided, other options describe different market structures. For example, a market with many sellers of a homogeneous product indicates perfect competition, where numerous firms offer identical products and none can influence the market price significantly. A scenario where a few firms control the market describes an oligopoly, characterized by a limited number of sellers who may set prices and output levels collaboratively. Finally, a competitive market without barriers to entry refers to a market structure where multiple firms can freely enter and exit, leading to competition that keeps prices low and efficient production.

Thus, the definition that accurately captures the essence of a monopoly is the one that highlights the dominance of a single seller controlling the supply in the market.