Which of the following is a key characteristic of monopolies?

A key characteristic of monopolies is the control exerted by a single entity over the market prices. This occurs because, in a monopoly, there is only one producer or supplier that dominates the entire market. As a result, the monopolist can influence prices and output levels without competition. This lack of competition allows the monopolist to maximize profits by setting prices above the competitive equilibrium level, leading to a situation where consumer choice is limited, and market efficiency is typically reduced.

In contrast, multiple companies controlling market prices would suggest a competitive environment, which does not align with the definition of a monopoly. Full competition in production implies that numerous firms are competing against each other, which is also contrary to the idea of monopolistic control. Lastly, abundant product variety typically exists in competitive markets where many firms offer diverse products, while a monopoly might limit variety to maximize profits. These factors highlight why the characteristic of a single entity controlling market prices defines monopolies distinctly.