Final answer:
A horizontal demand curve is not consistent with a monopoly, which has a downward-sloping demand curve. Monopolists always have marginal revenue less than the price due to the necessity to lower the price on all units to sell an additional unit. Other characteristics of a monopoly do include a single seller, marginal revenue less than price, and typically a U-shaped average total cost curve.
Step-by-step explanation:
The item that is NOT consistent with a monopoly is b. a horizontal demand curve. A monopoly characteristically has a downward-sloping demand curve, indicating that a monopolist can only sell more output by reducing its price. This is because the entire market demand curve becomes the monopolist's demand curve, as they are the sole seller, and the curve reflects the law of demand -- the higher the price, the lower the quantity demanded, and vice versa.
As for why a monopolist's marginal revenue is always less than the price, it involves the demand curve's downward slope. When a monopolist sells an additional unit, it must lower the price not just for that unit but also for all previous units sold, which causes marginal revenue to be less than the price. This relationship is captured by the marginal revenue curve lying beneath the demand curve.
As for the options given, a monopoly typically is characterized by a. a single seller, c. marginal revenue is less than the price, and often d. a U-shaped average total cost curve. The U-shaped cost curve indicates that there are economies of scale up to a point, followed by diseconomies of scale.