TCP only acknowledges bytes upto first missing byte in the stream(cumulative acknowledgements). Here given the segment with sequence number $74$ and $97$ is lost so the NAK send to the source is $74$ (the next expected sequence number).
How does the destination knows that the next missing byte is 74 only. It could be 51 also as there is no mention of amount of data bytes sent ??..plz ans.
if the first and fourth segments arrive at destination successfully