The correct answer is B. Downward.
A demand curve is a graph that shows the relationship between the price of a good and the quantity demanded of that good. The demand curve slopes downward because, as the price of a good decreases, consumers are willing and able to purchase more of that good.
Option A is incorrect because an upward-sloping demand curve would mean that consumers are willing and able to purchase more of a good as the price of that good increases. This is not the case for most goods and services.
Option C is incorrect because a horizontal demand curve would mean that consumers are willing and able to purchase the same quantity of a good regardless of the price of that good. This is also not the case for most goods and services.
Option D is incorrect because a vertical demand curve would mean that consumers are not willing or able to purchase any of a good at any price. This is also not the case for most goods and services.