I am frustrated with the following question , and need your help .
--. _________ is her elder brother ? --The man repairing the car over there.
A. Who B. Which one
I think “who” is the best choice for the gap because it is simple and easy to say ,but my friend says “which one” is the best choice for the gap .I really don’t know which one is the normal way the native speakers use in this situation .
Adding to SG's reply, both (A) Who and (B) Which one are possible answers (which is why you are confused), but when you take into consideration that the exam question is testing your knowledge on the usage of relative pronouns who and which, you'll be able to gain a better understanding why (B) Which one is the best answer.
Learners are taught that who refers to people and that which refers to things and so many test-takers will choose (A) Who as the answer to the exam question; but, and as SG points out, the correct answer is (B) Which one. The reason, which is not a relative pronoun in the phrase which one. It's a qualifier. It qualifies the pronoun one:
Q: Which one (of the men) is her elder brother?
A: The man repairing the car over there.
By the way, you don't need to know the context to answer the exam question correctly: you don't need to know that there's a choice of men. If that were the case, then Who would be just as likely an answer as Which one, right? (for example, Who is her elder brother, Mihir or Sanjit?). All you need to know is that which is not a relative pronoun in which one and for that reason it can indeed be used to refer to people.