@Deepalitrapti
A) Receiver can handle maximum 12 segments. So sender can send maximum 12 segments (if we can not consider network's capacity if network capacity is given then sender can send $Min(Receiver's capacity , Network's capacity)$).
B) Threshold is 4 .. means start with 1 and grow exponentially till 4 , after that grow linearly.
So it sender send pkt like this
1 | 2 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | 11 | 12
So total 10 RRT needs to reaches this window size, then time is
$ 10 * RTT = 10*100ms = 1000ms = 1sec$