The correct answer is C. Horizontal.
A perfectly elastic demand curve is a theoretical concept that describes a situation in which consumers are willing and able to buy any quantity of a good or service at a given price. In other words, the demand for the good or service is completely unresponsive to changes in price.
A horizontal demand curve is a graphical representation of perfectly elastic demand. The curve is perfectly horizontal, which means that the quantity demanded is the same at all prices.
A downward-sloping demand curve is a graphical representation of normal demand. The curve slopes downward from left to right, which means that consumers are willing and able to buy more of a good or service at a lower price.
None of these is the correct answer because it does not describe a demand curve.