Answer
Option ($D$).
Simplified form: $\dfrac{\sqrt3}{2}$
Work Step by Step
A radical expression is not yet simplified when:
(1) there is a radical in the denominator; or
(2) the radicand still has a factor that is a perfect root of the index.
Note that the radical expression in Option (D) has a denominator inside the radical sign. Thus, it is not yet in simplified form.
To simplify the expression in $D$, factor each radicand so that one factor is a perfect square to obtain:
$=\sqrt{\dfrac{3}{2^2}}$
Using the rule $\sqrt{\dfrac{a}{b}}=\dfrac{\sqrt{a}}{\sqrt{b}}. a\gt0, b\gt0$, the expression above simplifies to:
$=\dfrac{\sqrt{3}}{\sqrt{2^2}}
\\=\color{blue}{\dfrac{\sqrt{3}}{2}}$
