Final answer:
The profit-maximizing choice for a perfectly competitive firm occurs when marginal revenue (MR) equals marginal cost (MC), which is represented by the equation MR = MC.
Step-by-step explanation:
The profit-maximizing choice for a perfectly competitive firm occurs when marginal revenue (MR) equals marginal cost (MC), which is represented by the equation MR = MC. In the given figure, the profit-maximizing level of output is Q = 80. This means that at Q = 80, the firm is producing the quantity of output that will maximize its profits.