a
Electric force $\mathrm{Fe}=\mathrm{kq}_{1} \mathrm{q}_{2} / \mathrm{r}^{2}=\mathrm{kq}^{2} / \mathrm{r}^{2}$
Magnetic force $=\mathrm{qv}\left(\frac{\mu_{0}}{4 \pi} \times \frac{\mathrm{qv}}{r^{2}}\right)$
$\mathrm{F}_{\mathrm{m}}=\mathrm{q}^{2} \mathrm{v}^{2}\left(\frac{\mu_{0}}{4 \pi}\right) \frac{1}{\mathrm{r}^{2}}$
$\frac{F e}{F_{m}}=\frac{k}{v^{2}\left(\frac{\mu_{0}}{4 \pi}\right)}=\frac{1}{v^{2} \varepsilon_{0} \mu_{0}}=\frac{c^{2}}{v^{2}}$