The correct answer is B. Wardha asked her sister how long she intended to stay there.
A is incorrect because it uses the word "that", which is used to introduce a clause. In this case, the speaker is not introducing a clause, but is simply asking a question.
C is incorrect because it uses the preposition "to". The preposition "to" is used to indicate a destination, but in this case, the speaker is not asking about a destination, but about a length of time.
D is the same as B, but with an unnecessary comma.
