The correct answer is D. All of the above.
In a perfectly competitive industry, there are many firms producing identical products. Firms are free to enter and exit the industry, and there is perfect information about prices and costs. In the long run, all firms in a perfectly competitive industry will earn zero economic profits. This is because, in the long run, firms will enter the industry if there are positive economic profits, and exit the industry if there are negative economic profits. This process of entry and exit will continue until economic profits are zero.
In the long run, a perfectly competitive firm will produce a level of output where long-run marginal cost is equal to long-run average cost. This is because, in the long run, a firm will expand its output until the marginal cost of production is equal to the price of the product. The price of the product is equal to the average revenue of the firm, which is equal to the average cost of production when the firm is producing at the level of output where long-run marginal cost is equal to long-run average cost.
Therefore, when a perfectly competitive industry is in long-run equilibrium, all firms in the industry will earn zero economic profits and produce a level of output where long-run marginal cost is equal to long-run average cost.