Which of the following is NOT an assumption of a monopoly?

A monopoly is characterized by several key assumptions that define its market structure. The correct choice, indicating what is NOT an assumption of a monopoly, is the availability of many close substitutes.

In a monopolistic market, a single firm dominates the industry, which means there are no close substitutes for its product. This lack of competition allows the monopolist to set prices above marginal costs and operate without the pressures posed by rival businesses. Thus, the existence of close substitutes would directly contradict the definition of a monopoly, as it would imply more competition and choice for consumers.

The other aspects are indeed assumptions of a monopoly. For example, the presence of one firm in the industry allows it to exert significant control over the market. Additionally, the objective of maximizing profits is fundamental to the behavior of monopolistic firms, as their pricing and production decisions are typically driven by a desire to enhance profitability. Lastly, barriers to entry, whether legal, economic, or strategic, prevent other firms from entering the market, further solidifying the monopolist's position.

Therefore, the assertion that there are many close substitutes available is correct because it doesn't align with the fundamental characteristics of monopoly power.