The majority of good books are fiction books, but not all fiction books are written by established authors.
We can infer that...
Not all good books are written by established authors.
There are some good books that are not non-fiction books.
The answer marked correct, that some good books are not fiction books, can only be inferred via fallacious reasoning. Most X's are Ys, therefore some X's are not Y. This is clearly a bogus deduction. But that's exactly the the type of reasoning at work to arrive at the purported answer to this question.