Question

what will be the angle of reflection if a ray of light strikes a plane mirror at an angle of 90

Answer 1

Zero degree, because the light Ray strikes the mirror normally i.e. angle of incidence is equal to angle of reflection=0 degree (By Laws of Reflection of Light)

Answer 2

Hence, angle of incidence = angle of reflection = 30 degrees.

The incident ray will have an angle of reflection of 30 degrees (made with a surface normal to the mirror surface).

The reflected ray will make an angle of 60 degrees (90 - 30 degrees) with the mirror surface.