Transmission TIme = $\frac{Frame\,size}{Bandwidth} =\frac{4096}{64*10^{3}}=64\,ms$
Round Trip Time = $2*Propogation\,delay = 2*256 = 512\,ms$
For maximum throughput,
$Window\,size = 1+\frac{Round\,Trip\,Time}{Transmission\,time}=1+\frac{512}{64}=9$
As, window size is $15\geq9$, So the throughput will be maximum
Throughput = Bandwidth = 64 kbps
Correct me if I'm wrong