The correct answer is: C. The demand curve is a downward curve.
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 or service increases, consumers will have less money to spend on other goods and services, so they will demand less of the good or service whose price has increased.
The demand curve is a graphical representation of the law of demand. It shows the relationship between the price of a good or service and the quantity demanded of that good or service. The demand curve is always downward-sloping, which means that the quantity demanded of a good or service decreases as the price of that good or service increases.
Option A is incorrect because the demand curve is always downward-sloping, not upward-sloping.
Option B is incorrect because the demand curve is a curve, not a straight line.
Option D is incorrect because the demand curve is not a vertical line. A vertical line would mean that the quantity demanded of a good or service is not affected by the price of that good or service, which is not the case.