Choose the correct answer in the following : let $\overrightarrow a$ and $\overrightarrow b$ be two unit vectors and θ is the angle between them. Then $\overrightarrow a\ + \overrightarrow b$ is a unit vector if

Question

$$\begin{array} (A)\; \theta = \frac{\pi}{4} \quad & (B)\; \theta = \frac{\pi}{3} \quad & (C) \;\theta = \frac{\pi}{2} \quad &(D) \;\theta = \frac{2\pi}{3} \end{array}$$