b
Potential difference in the circuit $=24-12=12$ $\mathrm{volt}.$
This potential difference is divided among two capacitors $\mathrm{C}_{1}$ and $\mathrm{C}_{2}$ in the inverse ratio of their capacities (as they are joined in series).
$\therefore $ $\mathrm{V}_{1}=\frac{\mathrm{C}_{2}}{\mathrm{C}_{1}+\mathrm{C}_{2}} \mathrm{V}=\frac{4}{2+4} \times 12=8 \mathrm{\,volt}$
As plate of capacitor $\mathrm{C}_{1}$ towards point $\mathrm{B}$ will be at $+$ $\mathrm{ve}$ potential, hence
${{\rm{V}}_{\rm{B}}} - {{\rm{V}}_{\rm{A}}} = 8{\mkern 1mu} \,{\rm{volt}}$ or
$\mathrm{V}_{\mathrm{A}}-\mathrm{V}_{\mathrm{B}}=-8 \mathrm{\,V}$