Explanation

(d) Electrostatic force = centripetal force
$\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \frac{1}{4\pi\varepsilon_{0}}\frac{{Ze}^{2}}{r^{2}} = \frac{{mv}^{2}}{r}$
$\therefore\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ v = \sqrt{\left( \frac{1}{4\pi\varepsilon_{0}}\frac{{Ze}^{2}}{mr} \right)}$
=$\sqrt{\frac{9\ \times \ 10^{9} \times \ {(1.6\ \times 10^{- 19})}^{2}}{\left( 9.1\ \times \ 10^{- 31} \right)\ \times \ (0.1\ \times 10^{- 9})}}$
$\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ = \ 1.59\ \times \ 10^{6}ms^{- 1}$