1. affect the quantity demanded in the same direction if the good is a normal good.
2. affect the quantity demanded in opposite directions if the good is an inferior good.
3. affect the quantity demanded in a direction that is unrelated to whether the good is complementary with certain other goods.
4. do all of the above.
Choose the option that yields the correct answer.
Option 4 is the correct one. The substitution effect of a fall in price is to increase the quantity demanded of the good under all circumstances. That fall in price also has the effect of increasing the purchasing power of consumers' money income. This will result in a further increase in the quantity demanded if the good is a normal good and a moderation of the increase in the quantity demanded if the good is inferior. In fact, in very extreme cases the combined income and substitution effects may bring about a net reduction in the quantity demanded when the price falls and the good is inferior.
Whether or not a good is complementary with certain other goods has nothing to do with the direction of the income or substitution effects of a change in its own price on the amount of it demanded. All that complementarity with another good implies is that if there is a fall in a good's own price and the quantity of it demanded therefore increases, the quantity demanded of any good that is complementary with it will also increase at every level of that complementary good's price.
Recall that the substitution effect is always negative because consumers tend to use the cheapest goods or combinations of goods to satisfy their wants---that enables them to purchase the largest desired bundle of goods and satisfy their wants to the greatest possible degree. Accordingly, when the price of a good falls, holding the prices of other goods constant, it will become a cheaper source of satisfaction of the particular wants that good satisfies. The consumer will therefore consume more of that good and less of those (now relatively more expensive) goods that are substitutes in satisfying the same wants.