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16. An increase in equilibrium price and a reduction in equilibrium quantity of paperback books, would have to be caused by a shift back in supply, as shown to the right. As we consider the choices below, we look for factors that shift back supply and nothing that affects demand.

16. Using simple supply and demand analysis, what is the most likely explanation for an increase in equilibrium price and reduction in equilibrium quantity of paperback books.

A decrease in the popularity of recreational reading. A reduction in the popularity of recreational reading, will lead to a shift back in the demand for paperback books. This, in turn, will cause price and quantity to both fall.
A decrease in the retail price of hardback books. Since hardback books are a substitute for paperbacks, as they become less expensive the demand for paperback books will fall; causing price and quantity to both fall. Alternatively, if we look at hardbacks as another product produced by the same firms that produce paperbacks, a reduction in the price of another produced good will shift out the supply of paperbacks, causing price to fall and quantity to rise.
An increase in the cost of binding and printing paperback books. These are clearly variable costs, and as they increase the supply curve for paperback books will shift back; causing price to rise and quantity demanded to fall.
An increase in the number of books made into movies. This could be argued to either increase or decrease the demand for paperback books but it won't have anything to do with the supply relationship, it's clearly a demand issue.
An increase in the popularity of movies. This may or may not affect the demand for paperback books. Most likely it will cause demand to fall, causing both price and quantity to fall. In any case, it shouldn't affect the supply relationship.