The correct answer is: C. AR = MC.
A firm’s monopoly power is the ability of a firm to raise its price above marginal cost without losing all of its customers. A firm with monopoly power is a price maker, meaning that it can set the price of its product. A firm with perfect competition, on the other hand, is a price taker, meaning that it must accept the market price.
The condition for a firm to have monopoly power is that its average revenue (AR) curve must be above its marginal cost (MC) curve. This means that the firm can charge a price above marginal cost and still sell some units of its product.
The answer A, AR = AC, is not correct because it does not imply that the firm has monopoly power. A firm can have AR = AC and still be a price taker if the demand curve is perfectly elastic.
The answer B, MR = MC, is not correct because it is the condition for a firm to be in a state of long-run equilibrium. A firm can be in a state of long-run equilibrium and still have monopoly power if the demand curve is not perfectly elastic.
The answer D, None of these, is not correct because one of the answers, C, is correct.