Answer will be D)
Sender window Size will be =Sequence Number of Last Frame Send- Sequence number of last acknowledgement received
Receiver Window Size=Sequence Number of largest accepted Frame- Sequence number last frame received
Sequentially acceptance of frame required here. Because otherwise, we cannot locate, where there is any lost frame or corrupted frame
Below picture is for SR protocol which clears how SWS and RWS is changing with respect to sending of packets and receiving acknowledgement
In the diagram Sender window size will not change until 1st ACK is received
Similarly, receiver will not change it's window size, if there is lost frame in middle. When lost frame received , then only receiver will slide it's frame buffer to it's next frame