"Additive Increase, Multiplicative Decrease Out of the three versions of TCP, the Reno version is most common today. It has been observed that, in this version, most of the time the congestion is detected and taken care of by observing the three duplicate ACKs. Even if there are some time-out events, TCP recovers from them by aggressive exponential growth."
This is taken from Data Communications and Networking by Forouzan. 5th edition. Page 784.
So in my understanding:
By the end of
1st transmission: 2+2=4
2nd transmission: 8 (slow start threshold)
3rd transmission: 9
4th transmission: 10
During 5th a time-out occurs. In AIMD in time-out we go back to the initial MSS. Therefore:
5th transmission: 2
Then we do fast recovery in case of timeouts.
6th transmission: 4
7th transmission: 8 (ss threshold)
8th transmission: 9
9th transmission: 10
10th transmission: 11
Which we don't have in the options. So I am assuming the recovery is not FR and Addictive increase instead.
In that case, by the end of:
6th transmission: 3
7th transmission: 4
8th transmission: 5
9th transmission: 6
10th transmission: 7
Also according to Wikipedia, "In TCP, after slow start, the additive increase parameter a is typically one MSS (maximum segment size) per round-trip time, and the multiplicative decrease factor b is typically 1/2."
But on time out we don't do multiplicative decrease. Multiplicative decrease is in case of 3 duplicate acknowledgements.