If $a,b,c$ are non- Coplanar unit vector such that $a \times (b \times c) = \large\frac{b+c}{\sqrt 2}$, then the angle between a and b is

Question

$\begin{array}{1 1} \frac{3 \pi}{4} \\ \frac{\pi}{4} \\ \frac{\pi}{2} \\ \pi \end{array}$