When a firm is producing at the level of output that maximizes profit, the correct statement is:
Marginal revenue is equal to marginal cost.
At this point, the firm is maximizing its profit because it is producing the quantity of output where the additional revenue generated from selling one more unit (marginal revenue) equals the additional cost of producing that unit (marginal cost). If marginal revenue were greater than marginal cost, the firm could increase profit by producing more, and if marginal cost were greater than marginal revenue, the firm would maximize profit by reducing output.