Which of the following is a key characteristic of monopolies?

A key characteristic of monopolies is that a single entity controls market prices. In a monopoly, one company or organization has significant market power, meaning it is the sole provider of a particular product or service. This control allows the monopolistic entity to set prices without competition, as there are no close substitutes available for consumers. As a result, consumers have to accept the price set by the monopoly, which can often lead to higher prices and reduced output compared to more competitive market structures.

In contrast, the other options describe scenarios that are not consistent with monopolistic characteristics. Multiple companies controlling market prices would suggest a competitive market or oligopoly, where several firms have some control over prices. Full competition in production indicates a market with many buyers and sellers, which is the opposite of a monopoly. Lastly, abundant product variety usually exists in competitive markets where multiple firms provide different options, unlike a monopoly where the lack of competition typically results in limited product variety.