The angle between the reflected light and the mirror surface when a light ray is incident normally on the mirror's surface is :

According to the law of reflection of light, 
Angle of incidence = Angle of Reflection, 
where, the angle of incidence is the angle between the incident ray and the normal, and the angle of reflection is the angle between the reflected ray and the normal.

In the given example, the ray is incident normally, i.e along the normal.
$\therefore$ The angle of incidence = 0^o
$\therefore$The angle of reflection = 0^o.
$\therefore$The angle between the reflected ray and the surface of the mirror =  90^o.

Therefore, 90^o is the correct response.