a b c d

c

$ \text{Potential due to sphere of radius R_1 is }V_1 = \frac{KQ_1}{R_1}$ $ \text{Potential due to sphere of radius R_2 is }V_2 = \frac{KQ_2}{r}$ $\Rightarrow V = V_1 + V_2 = \frac{KQ_1}{R_1} + \frac{KQ_2}{r}$