Answer:
Marginal cost equals marginal revenue.
Explanation:
Profit maximization states that firms must operate at a level of output where marginal costs = marginal revenue.
If the firm produces less that that, they will not be making the maximum profit because they could still produce more and make more money.
If the produces more, they will be losing money because cost will be higher than revenue.
The spot on a graph where marginal costs equal marginal revenue is the "sweet spot" where profits are maximized.