Final answer:
The correct answer option a. In the long run, monopolies can continue to earn positive economic profits due to high barriers to entry. However, firms in perfect competition and monopolistic competition are driven towards zero economic profits as new entrants erode any short-term profits. The correct answer is a monopoly.
Step-by-step explanation:
The type of market structure can determine whether a firm can continue to earn positive economic profit in the long run. Monopolies, with their high barriers to entry and market control, can sustain positive economic profits. Monopolistically competitive firms can earn profits or losses in the short run; however, in the long run, they tend to move towards a zero economic profit outcome due to the entry and exit of firms in the market. Lastly, in perfect competition, firms cannot earn positive economic profits in the long run, as any short-term profits attract immediate competition, driving profits down to zero.
When discussing the response of firms in a monopolistically competitive market, the scenario is such that the entry of new competitors, attracted by the original firm's profits, leads to the original firm's demand curve shifting to the left. Consequently, the firm's profit-maximizing price and output levels will decrease as the economic profits are eroded by the competition. Ultimately, this process leads to a state where firms in monopolistic competition will not continue to earn positive economic profits in the long run, settling at a normal profit level.
Therefore, the correct option that can continue to earn positive economic profits in the long run is a monopoly.