The correct answer is: B. At the minimum point of the marginal cost curve.
A perfectly competitive firm is a firm that is small relative to the market, and therefore has no influence over the market price. The firm’s demand curve is therefore perfectly elastic, and the firm will produce at the point where marginal cost equals price. This is the point of minimum average cost, and the firm will make zero profit in the long run.
Option A is incorrect because the average variable cost curve is the sum of the average fixed cost curve and the average variable cost curve. The average variable cost curve will always be above the average cost curve, and the minimum point of the average variable cost curve will not be the point of minimum profit.
Option C is incorrect because the marginal cost curve will always intersect the average cost curve at a point above the minimum point of the average cost curve. This is because the marginal cost curve is always above the average cost curve, and the average cost curve is always increasing.
Option D is incorrect because the average fixed cost curve is always falling, and the minimum point of the average fixed cost curve will not be the point of minimum profit.