The correct answer is: A. A few dominant firms and substantial barriers to entry.
An oligopoly is a market structure in which a small number of large firms dominate the market. These firms have a great deal of market power and can influence prices and output. Oligopolies are often characterized by collusion, which is when firms cooperate to fix prices or output. This can lead to higher prices and lower output for consumers.
B. A few large firms and no entry barriers is not a correct answer because it does not mention the substantial barriers to entry that are characteristic of oligopolies.
C. A large number of small firms and no entry barriers is not a correct answer because it does not mention the dominance of a few large firms that is characteristic of oligopolies.
D. One dominant firm and low entry barriers is not a correct answer because it does not mention the existence of a few large firms that is characteristic of oligopolies.