A price taker in perfect competition faces a horizontal demand curve; a monopolist faces the market’s downward-sloping one. What does that difference imply about the cost of selling one more unit?

B. That the monopolist must cut the price on every unit to sell more

The gap between price and marginal revenue is the whole of monopoly theory. Because MR < P, the monopolist stops short of the competitive quantity, and the deadweight loss is what sits in that gap.