What is the pure surd form of  $\frac{2}{3}\sqrt{18 }$ ?

Given a surd as follows,
$\frac{2}{3}\sqrt{18 }$ .
We evaluate the given surd by transferring the fraction outside the radical inside and simplify as follows, 
$\begin{array}{c}\frac{2}{3}\sqrt{18 }=\sqrt{ \frac{2}{3}\times \frac{2}{3}\times 18 }\\ =\sqrt{ 2\times 2\times 2 }\\ =\sqrt{ 8 }\end{array}$ 
So, we can observe that any number is not outside the radical symbol.
Hence, $\sqrt{8}$ is the pure surd form of $\frac{2}{3}\sqrt{18 }$ . 
Hence, the correct option is $\sqrt{8}$.