I don't immediately see a reason why H couldn't be in Y, I think the last slot in y could be either h or n. But this question isn't asking if H couldn't be in Y, it's asking if H doesn't have to be in Y. This is a must be true except question, so the wrong answers all must be true, and the correct answer could be false. Indeed H could be in y, but it also could not be in y.
