In slow start phase, window size increases exponentially upon receiving 1 ack till threshold and after that collision avoidance phase start where window size is increased 1 mss after every acknowledgement till max window is reached.
Slow start : 1,2 4 ,8... threshold
Collision avoidance: threshold+1, threshold +2.....Max window size
Threshold = max transit window size/2
In given question Max window size is 12000B and therefore
Threshold = 6000
So the sequence of transit will be
1st 2000
2nd 4000 1 ack received
3rd 6000 1ack received
4th 8000
Hence, Answer is 8000 B