If $\overrightarrow a,\:\overrightarrow b\:and\:\overrightarrow c$ are mutually $\perp$ vectors of equal magnitude then the angle $\theta$ made by $\overrightarrow a+\overrightarrow b+\overrightarrow c$with any one of these 3 vectors is ?

Question

$\begin{array}{1 1} cos ^{-1}\large\frac{1}{\sqrt 3} \\ cos ^{-1}\large\frac{1}{ 3} \\ \large\frac{\pi}{2} \\ \large\frac{\pi}{4} \end{array} $