Final answer:
The firm may be incurring economic losses despite charging a price higher than marginal cost, as covering average total costs is necessary for earning economic profit. Additionally, the presence of positive economic profits may attract new firms, leading to increased competition and reduced profits, achieving zero economic profits in the long-run equilibrium.
Step-by-step explanation:
When a profit-maximizing firm in a monopolistically competitive market charges a price higher than marginal cost, it is not necessarily true that the firm must be earning a positive economic profit. The correct statement is that b) The firm may be incurring economic losses. This can occur when the price is above marginal cost, but not enough to cover average total costs.
In the long run, if the firm is earning positive economic profits, new firms may be attracted to enter the market due to the presence of profits, which would increase the competition, and lead to a reduction in the original firm's demand and profits. Eventually, this entry of new firms will push the prices down, and profits will become normal (zero economic profit) in the long-run equilibrium. Hence, the answer is also related to d) New firms will enter the market in the long run.