A ray of light is incident on a plane mirror at an angle of 30${}^o$. At what angle with the horizontal must a plane mirror be placed so that the reflected ray becomes vertically upwards?

The correct option is C 30${}^o$
When a light ray falls on a mirror at an angle 30${}^o$, then the reflected ray will make the same angle with the plane as shown in fig. (i)
In order to make the reflected ray vertical, the mirror should be rotated at an angle of 60${}^o$.
So, the mirror should be tilted by
$\frac{{60}^o}{2}={30}^o$ as can be seen in Fig(ii)