The relative permittivity of distilled water is 81. The velocity of light in it will be: (Given $$\mu_r = 1$$)

We know that the speed of an electromagnetic wave in any medium depends on the electric permittivity and magnetic permeability of that medium. The general relation is stated first:

$$v \;=\; \frac{1}{\sqrt{\mu\,\varepsilon}}$$

Here, $$\mu$$ is the absolute magnetic permeability and $$\varepsilon$$ is the absolute electric permittivity of the medium.

For convenience, these absolute quantities are expressed through their relative values with respect to free space. Thus we write:

$$\mu \;=\; \mu_r\,\mu_0, \qquad \varepsilon \;=\; \varepsilon_r\,\varepsilon_0$$

Substituting these into the relation for $$v$$, we get:

$$v \;=\; \frac{1}{\sqrt{\mu_r \mu_0 \,\varepsilon_r \varepsilon_0}} \;=\; \frac{1}{\sqrt{\mu_r\varepsilon_r}\;\sqrt{\mu_0\varepsilon_0}}$$

The factor $$\dfrac{1}{\sqrt{\mu_0\varepsilon_0}}$$ is the speed of light in vacuum, customarily denoted by $$c$$. Hence we write the well-known formula:

$$v = \frac{c}{\sqrt{\mu_r\,\varepsilon_r}}$$

Now we substitute the given data. For distilled water, the relative permittivity is

$$\varepsilon_r = 81$$

and according to the statement of the problem the relative permeability is

$$\mu_r = 1$$

Using $$c = 3.0 \times 10^{8}\;{\rm m\,s^{-1}}$$, we have

$$v \;=\; \frac{3.0 \times 10^{8}}{\sqrt{1 \times 81}} \;=\; \frac{3.0 \times 10^{8}}{\sqrt{81}}$$

Because $$\sqrt{81} = 9$$, this simplifies step by step as follows:

$$v = \frac{3.0 \times 10^{8}}{9} = 0.333\ldots \times 10^{8}$$

Writing $$0.333\ldots \times 10^{8}$$ in proper scientific notation gives

$$v = 3.33 \times 10^{7}\;{\rm m\,s^{-1}}$$

Hence, the correct answer is Option C.