What is the value of frequency at which an e.m wave must be propagated for the D- region to have a refractive index of $0.49 $ Electron density for D- layer is $10^9m^{-3}$

Question

$\begin{array}{1 1} 3.27 \times 10^5\;Hz \\327 \times 10^5\;Hz \\ 32.7 \times 10^5\;Hz \\ 0.327 \times 10^5\;Hz\end{array} $