Final answer:
If cookies and coffee are complements, a fall in the price of cookies would lead to an increase in both the equilibrium price and quantity of coffee as the demand for coffee rises.
Step-by-step explanation:
If cookies and coffee are complements, a decrease in the price of cookies will typically lead to an increase in the demand for coffee as consumers who are buying more cookies will likely want more coffee to go with them. According to the law of demand, the demand curve for coffee would shift to the right, resulting in a higher equilibrium quantity of coffee sold as consumers increase their purchase of coffee to pair with the cheaper cookies.
As for equilibrium price, the increased demand for coffee would generally push the price up since more people are willing to buy coffee at any given price, up to the point where the new equilibrium is found where the quantity supplied meets this higher quantity demanded.
Therefore, the correct answer to the student's question is a. Both the equilibrium price and quantity would increase.