b
For equilibrium along the plane

\(mg \sin \theta=\frac{1}{4 \pi \epsilon_0} \times \frac{ q _0^2}{\left( h \operatorname{cosec} 30^{\circ}\right)^2}\)

\(\therefore h ^2=\frac{1}{4 \pi \epsilon_0} \times \frac{ q _0^2}{ mg \operatorname{cosec} 30^{\circ}}\)

\(=9 \times 10^9 \times \frac{\left(2 \times 10^{-6}\right)^2}{0.02 \times 10 \times 2}\)

\(\therefore h =3 \times 10^4 \times \frac{2 \times 10^{-6}}{0.2}\)

\(=0.3\,m\)

\(=300\,mm\)