In GBN , N is sender window size . N is given 4 here .
K = Sequence number ..
and K > N always .
Now, in question it says , " that at time t, the next in-order packet that the receiver is expecting has a sequence number of k." ~ it means receiver expects k, so last 4 ACKs are k‐1, k‐2, k‐3, k-4 ( as window size is 4 , given ) .
That is if receiver is waiting for packet K, then it has received (and ACKed) packet (K− 1) and the (N− 1) packets before that.
Our requirement is : "all possible values of the ACK field in all possible messages currently propagating back to the sender at time t " ~ That means the range of packets that can already send by the sender .
So till now , receiver expect Kth packet next , means it already received up to (k-1)th packets and (N-1) packets are acknowledged.
Suppose , those N ACKs that receiver send , none of them yet received by the sender, then ACK messages with values of [ K−N to K− 1] may still be propagating back to the sender.
That means if receiver expect packet 5 and window size is 4 then it receive from (5-4) to (5-1) = 1 to 4 packets already. And sender also send 1 to 4 packets already .
Now the sender has sent packets [K − N, K− 1], it must be the case that the sender has already received an ACK for ( K−N − 1).
because to send packet (k-N) it need to receive acknowledge for packet (k-N-1) .
Now sender receive ACK for (K-N-1) means receiver send that ACK for (K-N-1) already .
Once the receiver has sent an ACK for ( K − N− 1) , it will never send an ACK that is less than (K −N − 1).
So that means all possible ACK values that is propagating from receiver towards sender , can range from (K −N − 1) to (K − 1) ....(i)
because receiver next expect K th packet so that range must be up to ( K-1 ) starting from (K-N-1) .
Now , in question the value of N is given , that is 4.
Put this value in (i) and we get (K-4-1) to (K-1) = (K - 5) to (K-1)
which is option 1.