Final answer:
Firms are most likely to make positive economic profits in the long run when they operate in a monopoly market because high barriers to entry prevent competition, unlike in monopolistically competitive or perfectly competitive markets, where competition drives profits down to zero.
Step-by-step explanation:
In the question of when firms are most likely to make positive economic profits in the long run, the most likely scenario is when a monopoly exists in the market. This is because significant barriers to entry in a monopolistic market prevent new competitors from entering, thereby allowing the monopolist to maintain positive economic profits. By contrast, in a monopolistically competitive market, while firms may earn positive economic profits in the short term, the entry of new competitors will eventually reduce those profits to zero in the long run as the demand curve for each firm shifts until it just touches the average cost curve at the long-run equilibrium.
Firms in a perfectly competitive market also cannot sustain positive economic profits in the long run, as the entry and exit of firms will drive economic profits to zero. When the market demand is perfectly elastic, firms are price takers and can only earn normal profits (zero economic profit) in the long run.