Consider an instance of TCP’s Additive Increase Multiplicative Decrease (AIMD)…

• Start: cwnd = 2 MSS, ssthresh = 8 MSS.

• Transmission 1 (end): slow start doubles → cwnd = 4.

• Transmission 2 (end): slow start doubles → cwnd = 8 (equals ssthresh, so next RTT will be congestion avoidance).

• Transmission 3 (end): congestion avoidance (+1) → cwnd = 9.

• Transmission 4 (end): congestion avoidance (+1) → cwnd = 10.

• Timeout occurs during 5th transmission: immediately set
  ssthresh = cwnd/2 = 10/2 = 5 MSS, and cwnd = 1 MSS.

• Transmission 5 (end, after timeout): cwnd = 1.

• Transmission 6 (end): slow start doubles → cwnd = 2.

• Transmission 7 (end): slow start doubles → cwnd = 4.

• Transmission 8 (end): doubling would give 8 which exceeds ssthresh = 5, so cwnd is set to ssthresh = 5 and we switch to congestion avoidance.

• Transmission 9 (end): congestion avoidance → cwnd = 6.

• Transmission 10 (end): congestion avoidance → cwnd = 7.

So the congestion window at the end of the tenth transmission = 7 MSS (option C).