Final answer:
A perfectly competitive firm should produce less to maximize profit when marginal revenue is less than marginal cost; with a decrease in price, the optimal level of output will also decrease.
Step-by-step explanation:
In a perfectly competitive market, a firm finds its profit-maximizing level of output where marginal revenue (MR) is equal to marginal cost (MC). If a firm is producing output where MR is less than MC, it should produce less to increase its profits. Reducing output would continue until MR equals MC.
If the price of output decreases, this would mean the MR has also decreased since, in perfect competition, MR equals price. As a result, the firm's optimal level of output would decrease as well to maintain the condition where MR equals MC.