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.