The answer is c. A competitive firm's marginal revenue curve is horizontal; a monopolist's marginal revenue curve downward sloping because the competitive firm's marginal revenue is constant and equal to the price level whereas, in a monopoly market, the price is charged higher than the marginal revenue, therefore, the curve is downward sloping. Option A is incorrect because a competitive firm can choose its level of output but cannot choose its price level. Option b is also incorrect because a competitive firm's short-run profit is not zero and a competitive firm can earn a positive economic profit in the short-run.