In computer networks, goodput is the application level throughput, i.e. the number of useful information bits delivered by the network to a certain destination per unit of time. (From wikipedia).
So, successful delivery of packet can be assured if $\text{ACK}$ has been received for it.
So till time $'i'$ we would have transmitted $'i'$ packets but only $(i-N)$ can be acknowledged as minimum time for a packet to get Acknowledged is $N$ (since $\text{RTT}$ is $N$ which is equal to the window size, there is no waiting time
for the sender).
So, successfully delivered packets $=(i-N)$
Time for transmission $=i$
Goodput $=\dfrac{\text{Successfully delivered data}}{\text{Time}}$
$=\dfrac{(i-N)}{i}$
$= 1- \dfrac{ N}{i}$
Therefore (A)