b
In steady state, potential across $C_{1}=$ potential across $C_{2}$
i.e, $\frac{Q_{1}}{C_{1}}=\frac{Q_{2}}{C_{2}}=\frac{Q_{2}}{2 C_{1}} \Rightarrow Q_{2}=2 Q_{1}$
Also total charge, $Q=Q_{1}+Q_{2}=Q_{1}+2 Q_{1}=3 Q_{1}$
$\therefore Q_{1}=\frac{Q}{3}, Q_{2}=\frac{2 Q}{3}$