A circle with centre O is inscribed in a square whose side is 6

A circle with centre O is inscribed in a square whose side is 6. AC = BD is the diameter of the circle. If \angle AOB = 15^{\circ}, the area of the shaded region is

A: \frac{3\pi}{8}

B: \frac{3\pi}{4}

C: \frac{\pi}{4}

D: \frac{3\pi}{5}

E: \frac{2\pi}{3}