The correct answer is D. All of the above.
A demand curve is a graphical representation of the law of demand, which 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. The demand curve for a normal good slopes downward, which means that there is a negative relationship between price and demand. This is because consumers are generally willing to purchase more of a good or service when the price is lower.
In addition, the demand curve for a normal good explains the relation between price and demand. The demand curve shows how much of a good or service consumers are willing and able to purchase at different prices. The downward slope of the demand curve indicates that consumers are willing to purchase more of a good or service when the price is lower.