Round trip delay $= 80\ ms$.
Quoting from Wikipedia
the round-trip delay time (RTD) or round-trip time (RTT) is the length of time
it takes for a signal to be sent plus the length of time it takes for an acknowledgment
of that signal to be received.
Now, in many books including standard ones, they have used $\text{RTT}$ to mean just the $2\text{-way}$ propagation delay by considering the signal/packet as of the smallest possible quantity so that its transmission time is negligible. The given question is following the first definition as given by Wikipedia which is clear from the choices.
During this time the first $\text{ACK}$ arrives and so sender can continue sending frames.
So, for maximum utilization sender should have used the full bandwidth during this time.
i.e., it should have sent $128\text{ kbps}\times 80\ ms$ amount of data and a packet being of size $32\text{ bytes},$ we get
no. of packets $=\dfrac{128 \times 80}{32 \times 8} = 40.$
Correct Answer: $B$