given sequence number =3

so window size of sender and receiver = 4 ...(2^3/2)

sender sends 0 then 1 then 2 then 3

but data packet 2 is lost

so receiver gets packets 0 , 1 and 3 but since 3 is an out of order packet so it sends NACK for packet 2 to the receiver.

hence the next 4 packets that the receiver is expecting is 2 , 4 , 5 ,6.

So D.

so window size of sender and receiver = 4 ...(2^3/2)

sender sends 0 then 1 then 2 then 3

but data packet 2 is lost

so receiver gets packets 0 , 1 and 3 but since 3 is an out of order packet so it sends NACK for packet 2 to the receiver.

hence the next 4 packets that the receiver is expecting is 2 , 4 , 5 ,6.

So D.