Final answer:
An increase in the price of textbooks, which are complements to study guides, will lead to fewer study guides being sold because the demand for both goods is linked.
Step-by-step explanation:
When two goods are complements, it means that the consumption of one good tends to enhance the consumption of the other. For instance, textbooks and study guides are often used together to help students learn course material more effectively. If the price of textbooks increases, students are likely to purchase fewer textbooks due to the higher cost. As a consequence, they will also need fewer study guides since they use these to supplement the textbook material. Therefore, if the price of textbooks goes up, the demand for study guides will generally decrease because the goods are complementary.
In response to the original question, the correct answer is:d. fewer study guides being sold. The increase in the price of textbooks will generally lead to a decrease in the demand for study guides because students are buying fewer textbooks and, hence, feel less need for study guides that are meant to complement those textbooks.