From the text, we can learn that medieval books were closely related to the process in which they were made, primarily because they were handcrafted by monks in monasteries. This labor-intensive process meant that most books were about religious topics, reflecting the intellectual and cultural limitations of the time. Since monks were the scribes who copied each book by hand, the subject matter was often confined to what was relevant to their religious communities, thereby producing a predominance of religious texts.
Thus, the correct response would be:
"Monks in monasteries made the books, so they were usually about religion."