I never read 'The Beast', but I'd like to say that it is most likely D. adult offenders who abuse juvenile offenders in prison.
Explanation:
"Beasts" are usually something that is cruel and harsh, which adult offenders who abuse juvenile offenders can be seen that way, and that seems much worse compared to the other options. Yet, since I assume you have read the story, I do advise you to think a bit deeper on the question if D. does not seem logical compared to the story.
