The correct answer is D. All of the above.
A competitive firm maximizes profit at the output level where marginal revenue equals marginal cost. This is because the firm’s profit is equal to the difference between its revenue and its costs. When marginal revenue equals marginal cost, the firm is producing at the point where the additional revenue it receives from selling one more unit of output is equal to the additional cost it incurs in producing that unit of output. This is the point at which the firm is making the most profit.
The slope of the firm’s profit function is equal to zero at the output level where marginal revenue equals marginal cost. This is because the profit function is a measure of the firm’s total profit, and the slope of the profit function is equal to the change in the firm’s profit divided by the change in the firm’s output. When the firm is producing at the point where marginal revenue equals marginal cost, the firm’s profit is not changing, so the slope of the profit function is equal to zero.
Price equals marginal cost at the output level where marginal revenue equals marginal cost. This is because the firm’s marginal revenue is equal to the price it receives for each unit of output it sells. When the firm is producing at the point where marginal revenue equals marginal cost, the firm is selling its output at the price that maximizes its profit.