Timer for frame 5 is expired which means either it did not receive by the receiver or did not may be acknowledgement lost .
In go back N we resend the window if the timer is expired.
So we have send till 9th frame. so we resend all the frames in the window from 5 to 9.
5th frame is lost we have to send it anyway so only option D have 5 as viable option so correct answer is D.
