Choice "C" is correct. Any business firm that has the ability to control the price of the product it sells faces a downward-sloping demand curve for the firm. Only the firm in a competitive market is a price-taker facing a horizontal demand curve at the market equilibrium price.
Choice "b" is incorrect. Firms in competitive industries have no entry or exit barriers and are price-takers.
Choice "a" is incorrect. A firm controlling the price of the product it sells would not cause a horizontal supply curve.
Choice "d" is incorrect. Only firms in perfectly competitive markets (price-takers) face horizontal demand curves.