b
Let the charge \(q\) is placed at a distance \(x\) from \(+9 e\) and inbetween charges \(+9 e\) and te

Using Coulomb's law,

\(\frac{K \cdot 9 e \cdot q}{x^2}=\frac{K \cdot q \cdot e}{(16-x)^2}\left\{F=\frac{K q_1 q_2}{r^2}\right\}\)

\(\left(\frac{16-x^2}{x}\right)^2=\frac{1}{9}\)

\(\frac{16}{x}-1=\pm \frac{1}{3}\)

\(\Rightarrow \frac{16}{x}=\frac{4}{3} \text { or } \frac{2}{3}\)

\(\Rightarrow x=\frac{3 \times 16}{4} \text { or } \frac{3 \times 16}{2}\)

\(\therefore x=12\,cm \text { or } 24\,cm\)

As the charge \(q\), can be equilibrium present only when its is kept inbetween charges \(+9 e\) and \(+e\), correct answer is \(x=12\,cm\)