When the price of hot dogs falls, the demand for hot dogs will increase. This is because hot dogs and hot dog buns are considered complementary goods, meaning they are often consumed together. As more people decide to purchase hot dogs due to the lower price, the quantity of hot dog buns demanded will also rise to accompany the increased consumption of hot dogs.
So, the correct answer is: d) the demand for hot dog buns will increase. As hot dogs become cheaper, more consumers buy them, thus increasing the overall demand for hot dog buns to complement their purchases of hot dogs.