The correct answer is A. perfectly elastic demand curve.
A perfectly elastic demand curve is a horizontal line that indicates that the quantity demanded of a good will change infinitely in response to even a small change in price. This is because in perfect competition, there are many firms selling identical products, so each firm has no control over the price of its product. If a firm raises its price, it will lose all of its customers to other firms that are selling the same product at a lower price. Conversely, if a firm lowers its price, it will attract customers from other firms that are selling the same product at a higher price.
A perfectly inelastic demand curve is a vertical line that indicates that the quantity demanded of a good will not change at all in response to a change in price. This is because the good is a necessity, and consumers will buy the same amount of it regardless of the price.
A perfectly elastic supply curve is a horizontal line that indicates that firms will supply any quantity of a good at a given price. This is because in perfect competition, firms have no costs of production, so they are willing to supply any quantity of the good at the market price.
A perfectly inelastic supply curve is a vertical line that indicates that firms will supply a fixed quantity of a good regardless of the price. This is because the good is a scarce resource, and firms cannot produce more of it even if the price is higher.