Computer Networks

Answer : D

Explanation :

Stop-and-wait ARQ (Automatic Repeat Request) is similar to stop - and - wait flow control protocol, but it includes retransmission of data in case of lost or damaged frames.
Both data and ACK frames are numbered alternately 0 and 1. But NAK frames are not numbered. If any error occurs the Receiver sends a NAK frame, it tells the sender to retransmit the frame last sent. Sender maintains a copy of frame until the Acknowledgement is back.