Jackie has a piece of land which was designed as shown in the figure. He wants to lawn the shaded area. $O$ is the center of the circle whose diameter is 4m and $OABC$ is a square. If the shaded area is $\frac{1}{3} $rd of the area of the square, then the side of the square is

Question

$\begin{array}{1 1} (a)\; \pi \sqrt{3} \\ (b)\; \sqrt{3 \pi} \\ (c)\; 3 \sqrt{\pi} \\ (d)\; 3 \pi \end{array}$