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 demand curve for each firm is perfectly elastic.
A duopoly is a market structure in which there are only two firms selling a good or service. In a duopoly, each firm has some control over the price of its product, but it is not a price maker. The demand curve for each firm is downward-sloping, but it is not perfectly elastic.
A monopoly is a market structure in which there is only one firm selling a good or service. In a monopoly, the firm has complete control over the price of its product. The demand curve for a monopoly is downward-sloping and relatively inelastic.
An oligopoly is a market structure in which there are a few firms selling a good or service. In an oligopoly, each firm has some control over the price of its product, but it is not a price maker. The demand curve for each firm is downward-sloping, but it is not perfectly elastic.
In conclusion, the demand curve is perfectly elastic in perfect competition. This is because each firm in perfect competition is a price taker, and has no control over the price of its product.