The maximum distance between any two nodes is between $D_{10}$ and $D_{9}$ i.e (1000+900)
So delay to cover this distance = $\frac{1000+900}{2\times 10^8}=9.5\,microsec$
Since the hub produces delay of $2.5\,microsec$, so delay due to this $=2\times2.5=5$
Total delay $=9.5\times 2+5=24\,microsec$
Let minimum size $=x$
$\frac{x}{10\times10^6}=24\times10^{-6}$
$x=240\,bits$
Correct me if I'm wrong