If $c$ is the speed of electromagnetic waves in vacuum, their speed in a medium of dielectric constant $K$ and relative permeability $ \mu_r$ is

Question

$\begin {array} {1 1} (a)\;v =\large\frac{\mu}{\sqrt{\mu_r}} K & \quad (b)\;v =c \sqrt{\mu_r}\: K \\ (c)\;v =\large\frac{c}{\sqrt{\mu_r}}\: K & \quad (d)\;v =\large\frac{K}{\sqrt{\mu_r}} c \end {array}$