b
\(\text { From figure, } T \cos \theta=m g.........(i)\)

\(T \sin \theta=\frac{k q^{2}}{x^{2}}.........(ii)\)

From eqns. \((i)\) and \((ii)\), \(tan \theta=\frac{k q^{2}}{x^{2} m g}\)

since \(\theta\) is small, \(\therefore \tan \theta  \approx \sin \theta  = \frac{x}{{2l}}\)

\(\therefore \quad \frac{x}{2 l}=\frac{k q^{2}}{x^{2} m g} \Rightarrow q^{2}=x^{3} \frac{m g}{2 l k}\) or \(q \propto x^{3 / 2}\)

\(\Rightarrow \frac{d q}{d t} \propto \frac{3}{2} \sqrt{x} \frac{d x}{d t}=\frac{3}{2} \sqrt{x} v\)

since, \(\frac{d q}{d t}=\) constant

\(\therefore v \propto \frac{1}{\sqrt{x}}\)