I don't have the book on me but it would guess the stimulus goes something like this.....if someone does X, then they deserve punishment. They concluded from this that because someone is punished, they must have done X. The answer choice says...Mistakes being sufficient (i.e. doing X) to justify punishment for being required to justify punishment. This would be correct because there could be many things that could warrant punishment besides doing X. The wording is funny though and I could understand how you would misinterpret it.
So I'm reading the LR Bible and this potential answer choice for mistakes in reversal makes no sense to me: "Mistakes being sufficient to justify punishment for being required to justify it." (page 376) Does that make any sense to anyone else?