The place where her father disappeared and the reason why he did are unknown to her

Although the form is awkward, the words themselves give us some idea as to what is being talked about in the sentence. It talks about a girl and the fact that she does not know where her father disappeared and why her father disappeared. The issue with the current form is that the reason something happened is itself the answer to 'why' that thing happened, so the use of the word 'why' with the word 'reason' is redundant, options A and D can therefore be ruled out. Option C can also be ruled out because it eliminates an important word, unlike the word 'why', the word 'where' plays an important role in the sentence. Option C implies that the place was made to disappear by the father; but even that, it fails to write in grammatically correct form. Option B does not have these errors and is therefore the correct answer.