Which one of the following options is correct?

During the slow-start phase, the congestion window size ($\text{cwnd}$) doubles every round-trip time ($\text{RTT} = 1\text{ ms}$) until it reaches the slow-start threshold ($\text{ssthresh} = 10000$ segments).

Let us determine the range of segments transmitted during each round (RTT):

• Round 1 ($0 \le t < 1\text{ ms}$): $\text{cwnd} = 1$, transmits segment 1. Total transmitted = $1$.

• Round 2 ($1 \le t < 2\text{ ms}$): $\text{cwnd} = 2$, transmits segments 2–3. Total transmitted = $1 + 2 = 3$.

• Round 3 ($2 \le t < 3\text{ ms}$): $\text{cwnd} = 4$, transmits segments 4–7. Total transmitted = $3 + 4 = 7$.

In general, at the end of Round $k$ ($t = k\text{ ms}$), the cumulative number of segments transmitted is:

Evaluating for subsequent rounds:

• After Round 10 ($t = 10\text{ ms}$): Total segments sent = $2^{10} - 1 = 1023$.

• In Round 11 ($10 \le t < 11\text{ ms}$): $\text{cwnd} = 2^{10} = 1024$ segments (which is below $\text{ssthresh} = 10000$). Segments numbered 1024 to 2047 are transmitted.

Since segment number 2000 lies in the range $[1024, 2047]$, its transmission starts during Round 11, where:

Hence, option (B) is correct.