Here we know that :
TCP is full duplex as far as data communication is concerned . Also acknowledgement is piggy-backed here i.e. acknowledgement along its own data is sent by the receiver to sender and vice versa in their own turns.
So
a) Sender sends data with sequence numbers (0 - 99) to receiver at the first step.
b) Receiver is free to send data beginning with other sequence number so sends data from sequence number (200-299)..And it sends the acknowledgement as well with the acknowledgement number as indicated by the next byte sequence number it is expecting from sender which is 100 in this case.So X = 100
c) So sequence number in the next round by the sender will be (100) i.e. sender will start transmitting from sequence number 100.At the same time it acknowledges receiver data and hence sends next expected byte sequence no from receiver which is 300 in this case.So Y = 100 , Z = 300
From "Computer Networking A Top Down Approcah by Kruose"
Hence D) should be correct option.
