The correct answer is A.
The law of demand states that, all other things being equal, the quantity demanded of a good or service will decrease as the price of that good or service increases. This is because, as the price of a good increases, consumers will have less money to spend on other goods and services, so they will demand less of the good.
The 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 from left to right, which means that as the price of a good decreases, the quantity demanded of that good increases.
Option B is incorrect because the demand curve does not slope upward from left to right. Option C is incorrect because the demand curve does not slope upward from right to left. Option D is incorrect because the demand curve is not parallel to the horizontal axis.