A key difference between monopoly and oligopoly is that: There are incentives to collude in monopoly, but not in an oligopoly. There are barriers to entry in monopoly, but not in an oligopoly. There is profit maximization in a monopoly but, no profit maximization in an oligopoly. There is one firm in a monopoly, but at least 2 in an oligopoly.

To analyze the key differences between monopoly and oligopoly, let's break down the statements provided: 1. **There are incentives to collude in monopoly, but not in an oligopoly.** - This statement is incorrect. In a monopoly, there is only one firm, so there is no need to collude. In an oligopoly, firms may have incentives to collude to maximize profits collectively. 2. **There are barriers to entry in monopoly, but not in an oligopoly.** - This statement is partially true. Monopolies typically have high barriers to entry, preventing other firms from entering the market. Oligopolies can also have barriers to entry, but they may not be as high as in a monopoly. 3. **There is profit maximization in a monopoly but no profit maximization in an oligopoly.** - This statement is misleading. Both monopolies and oligopolies aim to maximize profits, but the strategies they use may differ due to the number of firms and market dynamics. 4. **There is one firm in a monopoly, but at least 2 in an oligopoly.** - This statement is correct. A monopoly consists of a single firm that dominates the market, while an oligopoly consists of a few firms (at least two) that have significant market power. Based on this analysis, the most accurate statement that highlights a key difference between monopoly and oligopoly is: **There is one firm in a monopoly, but at least 2 in an oligopoly.**