For selective repeat ARQ, we know that sender window size=receiver window size
and total no number of sequence numbers=2N (where N is the sender window size)
Hence 0-255 would be consumed for sending 256 frames.Next 0-143 would be consumed for sending 144 frames hence total number of frames sent=256+144=400.Hence the sequence number for next frame should be 144.
the sequence no of 400th frame will be 143
but they are asking
sequence number of frame to be send after sending 400 th frame is ?
so answer will be 144