Answer:
The correct answer is A) demand curves always slope downward.
A normal good is a good for which the quantity demanded increases as consumer income increases, holding all other factors constant. The income effect and substitution effect both contribute to the downward slope of the demand curve, but they operate in opposite directions. The income effect reflects the fact that as a consumer's income increases, they can afford to buy more of the good at every price level, which increases the quantity demanded at each price level. The substitution effect reflects the fact that as the price of a good increase, consumers will switch to substitute goods, which decreases the quantity demanded at each price level. However, the net effect of these two effects is a downward-sloping demand curve.
The Engel curve, on the other hand, shows the relationship between the quantity of a good consumed and income, holding the price of the good constant. For a normal good, the Engel curve slopes upward, indicating that as income increases, the quantity of the good consumed also increases.