If the sequence number of a segment is m, then the sequence number of the subsequent segment is always m+1.
False. Sequence numbers indicate bytes, not segments.
If the estimated round trip time at any given point of time is t sec, the value of the retransmission timeout is always set to greater than or equal to t sec.
True. If RTO is set lower than RTT, Sender will have to retransmit even before the Receiver has a proper chance to acknowledge.
The size of the advertised window never changes during the course of the TCP connection.
False. Sender and Receiver's buffer sizes are fixed.
Congestion window and Advertised window are dynamic windows that change as per the current circumstances.
The number of unacknowledged bytes at the sender is always less than or equal to the advertised window.
Unacknowledged bytes consist of the segments that are currently in the Sender Window. (If they were acknowledged, they won't be in the Sender's window, because, obviously, they're transmitted successfully. Why would we keep them in the transmission (sender) window again?)
The amount of unacknowledged data at a sender can't exceed the minimum of $cwnd$ and $rwnd$, that is:
$$LastByteSent \ – \ LastByteAcked \leq min\{cwnd, rwnd\} $$
And, $cwnd$ can't exceed $rwnd$ (to maintain the flow control)
Hence, the size of $rwnd$ is the upper bound of the number of unacknowledged bytes at the sender.
So, True.
Option B