From the text, we learn that the process of making medieval books was highly labor-intensive and primarily carried out by monks in monasteries. This process involved multiple steps, including careful copying by scribes, error checking by rubicators, and illustration by illuminators. Because the majority of books were produced in this manner, they were predominantly about religious topics, as the intellectual and cultural life of the time was largely confined to religious communities. Therefore, the correct response would be:
Monks in monasteries made the books, so they were usually about religion.