The correct answer is A. Downward from left to right.
A monopoly is a market structure in which there is only one seller of a good or service. The monopolist faces the entire demand curve for the good or service, and can therefore charge any price it wants. However, if the monopolist charges a price that is too high, consumers will buy less of the good or service. The monopolist therefore has to balance the desire to charge a high price with the need to sell enough units to make a profit.
The average revenue curve for a monopoly is downward-sloping because the monopolist can charge a higher price for each additional unit sold. However, as the monopolist charges a higher price, it will sell fewer units. The average revenue curve therefore reflects the trade-off between price and quantity that the monopolist faces.
Option B is incorrect because the average revenue curve for a monopoly cannot be upward-sloping. If the monopolist could charge a higher price for each additional unit sold, it would always do so. This would lead to a situation in which the monopolist would charge a very high price for a very small number of units sold. This would not be a profitable situation for the monopolist.
Option C is incorrect because the average revenue curve for a monopoly cannot be a horizontally parallel straight line. If the average revenue curve were a horizontal line, this would mean that the monopolist could charge any price it wanted and still sell the same number of units. This is not possible, because if the monopolist charges a price that is too high, consumers will buy less of the good or service.
Option D is incorrect because the average revenue curve for a monopoly cannot be a vertical straight line. If the average revenue curve were a vertical line, this would mean that the monopolist could not charge any price it wanted. This is not possible, because the monopolist can always charge a higher price for each additional unit sold.