Correct : d

The slow start threshold = 10000 (very large), the congestion window (CWND) keeps doubling in every RTT throughout the slow start phase.
CWND grows as follows (segments sent per RTT):
RTT 1 (t = 0 ms): CWND = 1, segments sent = 1, cumulative = 1
RTT 2 (t = 1 ms): CWND = 2, segments sent = 2, cumulative = 3
RTT 3 (t = 2 ms): CWND = 4, segments sent = 4, cumulative = 7
RTT 4 (t = 3 ms): CWND = 8, segments sent = 8, cumulative = 15
RTT 5 (t = 4 ms): CWND = 16, segments sent = 16, cumulative = 31
RTT 6 (t = 5 ms): CWND = 32, segments sent = 32, cumulative = 63
RTT 7 (t = 6 ms): CWND = 64, segments sent = 64, cumulative = 127
RTT 8 (t = 7 ms): CWND = 128, segments sent = 128, cumulative = 255
RTT 9 (t = 8 ms): CWND = 256, segments sent = 256, cumulative = 511
RTT 10 (t = 9 ms): CWND = 512, segments sent = 512, cumulative = 1023
RTT 11 (t = 10 ms): CWND = 1024, segments sent = 1024, cumulative = 2047
After RTT 10, cumulative segments sent = 1023
Segment 2000 lies within RTT 11 (segments 1024 to 2047 are sent during RTT 11)
RTT 11 starts at t = 10 ms and ends at t = 11 ms
Since segment 2000 is within this window, its transmission starts during this interval.
The time t at which segment 2000 starts transmission satisfies 10 ≤ t < 11 (Option D)