The correct answer is: C. Substitution effect + income effect.
The substitution effect is the change in the quantity demanded of a good that results from a change in its relative price, holding real income constant. The income effect is the change in the quantity demanded of a good that results from a change in real income, holding relative prices constant.
The total effect of a price change is the sum of the substitution effect and the income effect. The substitution effect always leads to a decrease in the quantity demanded of a good when its price increases, while the income effect can lead to either an increase or a decrease in the quantity demanded of a good, depending on whether the good is a normal good or an inferior good.
For a normal good, the income effect will reinforce the substitution effect, leading to a larger decrease in the quantity demanded of the good when its price increases. For an inferior good, the income effect will work in the opposite direction of the substitution effect, leading to a smaller decrease in the quantity demanded of the good when its price increases.
In general, the substitution effect is always stronger than the income effect, so the total effect of a price change is usually a decrease in the quantity demanded of the good. However, there are cases where the income effect can be stronger than the substitution effect, leading to an increase in the quantity demanded of the good when its price increases.