Final answer:
When the price of movies goes down, C) the quantity demanded popcorn will increase because movies and popcorn are complementary goods.
Step-by-step explanation:
If popcorn and movies are complemented, and the price of movies goes down, the quantity demanded for popcorn will increase. This occurs because, with the reduction in the price of movies, more people are expected to go to the movies, thereby increasing the demand for popcorn which is consumed along with watching movies. It's important to differentiate between an increase in demand, which implies a shift in the demand curve, and an increase in quantity demanded, which is a movement along the demand curve due to a change in the price of the good itself or its complement.
So, the correct answer to the student's question would be C) The quantity demanded for popcorn will increase.