An ACK number of 1000 in TCP always means that
in TCP ,ack no. tells which value is next expected to be sent.. so all above options are wrong...
answer is option "d" none of these
If ISN = 1, then ACK number = 1000 would mean 999 bytes delivered.
If ISN = 0, then ACK number = 1000 would mean 1000 bytes delivered.
Option C is never possible because ISN can't be negative.
ACK number x simply means that the receiver now expects the xth byte.
Total delivered bytes OF A SESSION would be calculated by subtracting ISN from the ACK number. (New Session = New ISN)
Option D is correct.