d
\(\frac{\varepsilon_{\mathrm{II}} \times \mu_{\mathrm{mL}}}{\varepsilon_0 \times \mu_0}=\frac{\frac{1}{\mathrm{v}^2}}{\frac{1}{\mathrm{c}^2}}\)

\(\varepsilon_{\mathrm{r}} \times \mu_{\mathrm{r}}=\frac{\mathrm{c}^2}{\mathrm{v}^2}\)

\(\varepsilon_{\mathrm{r}} \times 2=\frac{\left(3 \times 10^8\right)^2}{\left(1.5 \times 10^8\right)^2}\)

\(\varepsilon_{\mathrm{r}} \times 2=4\)

\(\varepsilon_{\mathrm{r}}=2\)