The best sampling of readers for the author to use in a survey would be:
C. 5,000 middle school students in his city
This option is ideal because middle school students are a targeted demographic that often reads young adult books. It provides a direct insight into the preferences of the readers the author is likely trying to reach. The other options may include individuals who are not the primary audience for young adult books or may not represent a broad enough range of preferences among potential readers.