What will be the angle of reflection if a ray of light strikes a plane mirror normally?

The correct option is C

$0^{\circ }$

When Incident ray AI moves towards normal OI the reflected ray also moves closer to the normal.

As a result, the angle between normal and reflected ray (angle of reflection) decreases and becomes zero when incident ray overlaps normal.

or

For normal incidence,
Angle of incidence = $0^{\circ }$
We know,
The angle of reflection = Angle of incidence
Therefore, here the angle of reflection = $0^{\circ }$