Area of the largest circle that can be inscribed in a semicircle of radius ‘r’ units is

Question

$\begin{array}{1 1}(A) \; \sqrt 2 r^2 sq.units \\(B)\;r^2 sq.units \\(C)\;\large\frac{r^2}{\sqrt 2} sq.units\\(D)\; \large\frac{r^2}{2} sq.units \end{array}$