$V_c=\frac{3 k Q}{2 R}$
$V_s=\frac{k Q}{R+x}=\frac{3 k Q}{4 R}$
$4 R=3 R+3 x$
$3 x=R$
$x=\frac{R}{3}$



$(A)$ the charge on the upper plate of $C _1$ is $2 CV _0$
$(B)$ the charge on the upper plate of $C _1$ is $CV _0$
$(C)$ the charge on the upper plate of $C _2$ is $0$
$(D)$ the charge on the upper plate of $C _2$ is $- CV _0$
