A、Literature. B、Philosophy. C、Sociology. D、History. C在对话中，借阅人提到了他所需要的书的书名为Sociology and the Modern Age，因此答案为C

W: Can I help you?
M: Yes. I am a bit confused. My sociology class is supposed to read a chapter in a book called Sociology and the Modem Age. According to the syllabus, the book is in the library, but I haven’t been able to find it.
W: Do you have your syllabus with you? May I see it?
M: Yes, uh...I put it on the front of my sociology notebook. Oh, here it is.
W: Let me see. Oh yes. Your professor has placed this book on reserve. That means you cannot find it on the shelves in its usual place. You need to go to a special room called the reserve room. It’s down the hall and to the right.
W: I’m sorry—I still don’t understand what you mean by "on reserve".
W: You see, your professor wants everyone in the class to read the chapter. If one student removes the book from the library, it is likely that none of the other students will have the opportunity to read it. So, your professor has insured that all students have the opportunity to read it by placing it on reserve.
M: So, will I be able to find this book?
W: Yes, when a book is on reserve, a student can go to the reserve room and ask the reserve librarian for the book. The student can have the book for a few hours, and he or she MUST read it in the library during that time. That way, the book stays in the library, and all students have a chance to read it.
M: Okay. Thank you. I understand now.
W: Will there be anything else?
M: No! I am on my way to the reserve room. Thanks again!