The correct answer is A. Profit curve.
Average revenue (AR) is the total revenue divided by the number of units sold. It is calculated by taking the price of a good and multiplying it by the number of units sold. The average revenue curve is a graph that shows the relationship between the average revenue and the quantity of output produced.
The profit curve is a graph that shows the relationship between the total profit and the quantity of output produced. The profit is the difference between the total revenue and the total cost. The profit curve is always downward sloping, because as the quantity of output produced increases, the total cost increases at a faster rate than the total revenue.
The demand curve is a graph that shows the relationship between the price of a good and the quantity of the good that consumers are willing and able to buy. The demand curve is always downward sloping, because as the price of a good decreases, consumers are willing and able to buy more of the good.
The average cost curve is a graph that shows the relationship between the average cost and the quantity of output produced. The average cost is the total cost divided by the number of units produced. The average cost curve is U-shaped, because as the quantity of output produced increases, the average cost decreases at first, but then increases as the firm reaches its maximum efficiency level.
The indifference curve is a graph that shows the combinations of goods that a consumer is indifferent between. The indifference curve is convex to the origin, because a consumer would prefer to have more of both goods rather than less of both goods.