c
(c)Speed of light of vacuum \(c = \frac{1}{{\sqrt {{\mu _0}{\varepsilon _0}} }}\) and in another medium \(v = \frac{1}{{\sqrt {\mu \varepsilon } }}\)
\(\frac{c}{v} = \sqrt {\frac{{\mu \varepsilon }}{{{\mu _0}{\varepsilon _0}}}} = \sqrt {{\mu _r}K} \) ==> \(v = \frac{c}{{\sqrt {{\mu _r}K} }}\)