a b c d

d

$\text{At point of return V= 0 i.e K.E = 0}$ From Energy conservation K_i + U_i = K_F + U_f $ $\Rightarrow \frac{1}{2} mv^2 + \frac{kqQ}{\infty} = 0 + \frac{kqQ}{r} $ $ r = \frac{2kqQ}{mv^2} $ $ r \propto \frac{1}{r^2}$ $\text{If velocity is 2v then } r'= \frac{r}{4}$