Answer C),I am giving the counter example for other options
a)Let we have a sliding window protocol $(GB_{N})$ Window of size $4$.Sender will send $1,2,3,4$.Let packet number $2$ gets lost , so reciever will send Ack no $2$(that it is expecting) $3$ times even though sender will send packet $1,2,3,4$ .After that sender will send packet $2,3,4,5$
b)same as a) i,e even though sender will recieve ack no $3,3,3$(duplicate ack) , it(sender) will keep on sending packet of all its window i.e$1,2,3,4$.
c) correct
Any new (previosly unsent) packet with sequence number greater than $W$ is sent by sender if and only if a new (previosly unseen ) packet arrives.
It means sender cannot send packet i+w, until it has recieved ack for packet i where w=window size
