In long-run equilibrium,both purely competitive and monopolistically competitive firms will:

A) produce at minimum average total cost.
B) earn economic profits.
C) achieve allocative efficiency.
D) equate marginal cost and marginal revenue.

In long-run equilibrium, both purely competitive and monopolistically competitive firms equate marginal cost (MC) and marginal revenue (MR) to maximize profit. However, only purely competitive firms produce at minimum average total cost and achieve allocative efficiency in the long run, while monopolistically competitive firms do not due to product differentiation and excess capacity. Economic profits are also driven to zero in the long run for both market structures due to free entry and exit, but this is not directly related to the choices given.