a
In series, potential divides in direct ratio of resistance,

So, \(V_{A C}=\frac{R_{A C}}{R_{A C}+R_{C B}} V_0\)

\(=\frac{\frac{ RR _0}{4 R + R _0}}{\frac{ RR _0}{4 R + R _0}+\frac{3 R _0}{4}} \times V _0=\frac{4 RV _0}{16 R +3 R _0}\)