The speed of electromagnetic radiation in matter is

$\begin{align*}v = \frac{c}{n} = \frac{c}{\displaystyle \sqrt{\frac{\epsilon \mu}{\epsilon_0 \mu_0}}} = \frac{c}{\sqrt{\epsilo...$
$\epsilon_r$ , the relative permittivity (also known as the dielectric constant), is 2.1 and $\mu_r$ , the relative permeability, is 1.0, so

$\begin{align*}v = \frac{c}{\sqrt{2.1 \cdot 1.0}} = \boxed{\frac{c}{\sqrt{2.1}}}\end{align*}$
Hence, answer (D) is correct.

Solution to 1986 Problem 3