Explanation

(d) Coulomb force is given by
$F = \frac{1}{4\pi\varepsilon_{0}}\frac{q_{1}q_{2}}{r^{2}}$
$\therefore\left( 10 \times 10^{- 3} \right) \times 10 = \frac{(9 \times 10^{9}) \times q^{2}}{(0.6)^{2}}$
$(\because q_{1} = q_{2} = q)$
or $q^{2} = \frac{10^{- 1} \times 0.36}{9 \times 10^{9}}$
$\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ = 4 \times 10^{- 12}$
$\ \ \ \ \ \ \ \ \ \ \ q = 2 \times 10^{- 6}\complement = 2\mu\complement$