$\begin{align*} \int_0^a \sqrt{a^2 - x^2} dx \end{align*}$ is equal to :

Question

( A ) $\frac{\pi a^2}{3}$
( B ) $\pi a^2$
( C ) $\frac{\pi a^2}{4}$
( D ) $\frac{\pi a^2}{2}$