Final answer:
In the long run, monopolistic competitors will see a decrease in profit-maximizing price due to increased competition, which leads to zero economic profits and maintains output levels constant.
Step-by-step explanation:
In a monopolistically competitive market, in the long run, the profit-maximizing price and output levels for a firm will change due to market entry by new firms. Initially, a firm may earn positive economic profits; however, these profits attract new entrants into the market, which increases the overall competition. As competition increases, demand for the original firm's product decreases, leading to a decrease in both the firm's profit-maximizing price and output levels. Ultimately, the firm's economic profits are eroded, and it will reach a point where it earns zero economic profits in the long-run equilibrium. Therefore, the correct response to what happens to profit-maximizing price and output levels is (a) Price decreases, output remains constant.