Final answer:
Monopoly is a market structure with a single seller who has significant market power and the ability to influence prices, making option 1) Single seller the correct characteristic of a monopoly.
Step-by-step explanation:
A monopoly is a market structure characterized by a single seller. It is defined by the unique position of one producer with a unique product and very high barriers to entry in the market, which prevents other firms from entering the market. Since a monopoly has no significant competition, it holds a great deal of market power and can set prices at will, within the limits set by the demand curve. In contrast, an oligopoly is a market structure with very few producers, high barriers to entry, and significant, though not complete, market power held by each producer.
Therefore, relating to the student's question, option 1) Single seller accurately matches the characteristic of a monopoly market structure.