At what angle from horizontal must we keep the mirror in order to get a vertical reflected ray ?

The correct option is B ${30}^{\circ }$

Let suppose the mirror is rotated by $\theta$ in anticlockwise direction.

$\therefore$ Angle between old normal $N$ and new normal $N^{\prime }$ is $\theta$.

As per question, we want vertical reflected ray.

Angle of incidence with the new normal is ${60}^{\circ }-\theta$

$\therefore$ Angle of reflection is ${60}^{\circ }-\theta$

For vertical reflected ray,

$\theta ={60}^{\circ }-\theta$

$2\theta ={60}^{\circ }\Rightarrow \theta ={30}^{\circ }$

Hence, option $\left(b\right)$ is correct.