The correct option is C Ashu is calling for Ravi.
Option B: The conversation begins with Nishu answering a call: "Hello!"
The caller is Ashu, as he asks if he can speak to Ravi: "Hello! Can I speak to Ravi?"
So, through these lines, it is established that Ashu has called to speak to Ravi, he has called for Ravi. Hence option B is correct.
Option A: Since Ravi is out at that moment, his sister Nishu answers instead.
The lines "Nishu: I am his sister Nishu on the line. Ravi is out at the moment." convey that Nishu has not called for Ravi, but instead answered the call for him.
Option C: Nishu is not calling for Ashu, but she has answered Ashu's call meant for Ravi.
Hence options B and C are incorrect.
Option D is incorrect because the answer lies in B.