The correct answer is: B. downward.
In all forms of imperfect competition, the average revenue curve facing the individual firm slopes downward. This is because the firm can charge a higher price for its product if it produces a smaller quantity. As the firm produces more, the price it can charge will fall, because there will be more competition from other firms.
Option A is incorrect because the average revenue curve facing the individual firm slopes downward, not upward.
Option C is incorrect because the average revenue curve facing the individual firm is not parallel to the X-axis. It slopes downward.
Option D is incorrect because the average revenue curve facing the individual firm is not vertical. It slopes downward.