if $A = \begin{bmatrix} cos\alpha & -sin\alpha \\ sin\alpha & cos\alpha \end{bmatrix}$ then $A + A' = I,$ if the value of $\alpha$ is

Question

$$\begin{array}{1 1} (A) \; \frac{\pi}{6} & (B) \; \frac{\pi}{3} \\ (C) \; \pi & (D) \; \frac{3\pi}{2} \end{array}$$