When a monopolistically competitive firm is in long-run equilibrium:

A) P = MC = ATC.
B) MR = MC and minimum ATC > P.
C) MR > MC and P = minimum ATC.
D) MR = MC and P > minimum ATC.

In the long run, under monopolistic competition, a firm will produce at the point where MR = MC to maximize profits. However, due to the presence of differentiated products, the firm will not be able to charge the perfectly competitive price (minimum ATC). Therefore, in the long-run equilibrium, P will be greater than the minimum ATC, and equal to the firm's average revenue (AR). Thus, the correct choice is D, where MR = MC and P > minimum ATC.