The idea that was explicitly stated in the passage is that Mrs. Auld was doing something illegal.
From the information given, Mr Auld found out that Mrs Auld was teaching Frederick and he forbade her to instruct Frederick further, by telling her, that it was unlawful, and unsafe, to teach a slave to read. Unlawful means something that's illegal.
Yes, the answer is B. I took the test and got the question right. Also because the last sentence says, "...that it was unlawful, as well as unsafe, to teach a slave to read." Another defenition of unlawful is illegal. So, Mrs. Auld was doing something illegal by instructing a slave.
