Which best defines a monopoly?

A monopoly is best defined as a market dominated by a single seller. In this scenario, the monopolist has significant control over the market supply of a good or service, allowing it to set prices and dictate terms without facing competition from other sellers. This lack of competition typically leads to higher prices and lower output than would be found in more competitive market structures.

Monopolies can arise from various factors, including ownership of a vital resource, government regulation granting exclusive rights, or economies of scale that make it inefficient for multiple firms to coexist in the market. The absence of competitors means that the monopolist can influence market dynamics significantly, impacting consumers and the overall economy.

Other options describe different market structures or economic concepts. For instance, a market with many competitors would typically represent perfect competition, which stands in contrast to the monopolistic scenario. Government intervention, while it can relate to monopolies, does not define what a monopoly is. Similarly, a situation with perfect competition embodies numerous sellers and buyers, making it fundamentally different from a monopoly.