$1)$ In Go$-$Back$-$N ARQ protocol which uses a window size of $1000,$

the transmitter receives a negative acknowledgment $($REJ$900)$ after it

sent frame $950.$ In this case the transmitter must retransmit frames:

$($a$)900$ to $950($b$)1$ to $900($c$)900$ to $1000($d$)900$ only