Final answer:
The substitution and income effects both cause an increase in quantity purchased for a normal good when its price falls. The substitution effect encourages buying more of the cheaper good over others, while the income effect allows consumers to afford more due to increased purchasing power.
Step-by-step explanation:
When the price of a normal good falls, the substitution effect leads to an increase in the quantity purchased because the product becomes cheaper relative to other products, making consumers more likely to substitute other products with this now cheaper product. Simultaneously, the income effect also leads to an increase in the quantity purchased due to the effective increase in consumers' purchasing power, allowing them to buy more of the product with the same amount of income.
The correct answer is a. an increase; an increase. This is because, for normal goods, both the substitution effect and the income effect work in the same direction when the price of the good decreases. Consumers will tend to buy more of the cheaper good in place of others (the substitution effect) and they will also be able to afford more because they are effectively wealthier with the price decrease (the income effect).