This is my point of view I want to share.
In TCP AIMD, window size doubles or increases after successful acknowledgement. i.e; it is very much possible that 1 RTT (i.e; my minimum time we take to receive an acknowledgement) is greater than the transmission time of whole window. I meant to say that, it is possible, and most of the time happens, that once sender sends the whole window, it has to wait for sometime to receive an acknowledgement and increase its window size (whether doubles or single increment, depending on the phase going on).
Now coming to the question, when it says that at the end of 12th transmission, then we are talking only about transmission... i.e; the time when the whole window of the 12th transmission is sent. And as discussed earlier, there might be some time gap between the whole window transmission and 1Rtt, i.e; receiving and ACK. And until we receive an acknowledgement, how can the window size be adjusted?
So according to me 8 is the correct answer.