Explanation

(c) In vacuum, $\varepsilon_{0}$=1
In medium, $\varepsilon = 4$
So, refractive index
$\mu = \ \sqrt{\varepsilon/\varepsilon_{0}}\ = \ \sqrt{4/1}\ = 2$
wavelength $\lambda^{'}\ = \frac{\lambda}{\mu} = \frac{\lambda}{2}$
and wave velocity $\ v\ = \frac{c}{\mu} = \frac{c}{2}$
Hence, it is clear that wavelength and velocity will become half but frequency remains uncharged when the wave is passing through any medium.