The correct answer is: B. Perfect competition
In perfect competition, there are many firms selling identical products, and each firm is a price taker. This means that each firm has no control over the price of its product, and must accept the market price. As a result, the marginal revenue curve for a firm in perfect competition is equal to the price of its product.
In monopoly, there is only one firm selling a product. The firm has a great deal of market power, and can therefore charge a price above marginal cost. As a result, the marginal revenue curve for a monopoly firm is below the price of its product.
In monopolistic competition, there are many firms selling similar but not identical products. Each firm has some control over the price of its product, but not as much as a monopoly firm. As a result, the marginal revenue curve for a firm in monopolistic competition is below the price of its product, but above the marginal revenue curve for a firm in perfect competition.
In oligopoly, there are a few firms selling a product. The firms have some market power, but not as much as a monopoly firm. As a result, the marginal revenue curve for a firm in oligopoly is below the price of its product, but above the marginal revenue curve for a firm in perfect competition.