The correct answer is A. Horizontal.
A perfectly competitive firm is a price taker, meaning that it has no control over the price of its product. The demand curve for a perfectly competitive firm is therefore horizontal, or perfectly elastic. This means that the firm can sell as much or as little of its product as it wants at the market price.
A positively sloped demand curve indicates that a firm can charge a higher price for its product if it sells less of it. This is because the firm has some control over the price of its product. A negatively sloped demand curve indicates that a firm must charge a lower price for its product if it wants to sell more of it. This is because the firm has a limited amount of control over the price of its product.
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