Question

If the incident ray strikes the mirror at 90 degree what will be the angle of reflection?

Answer 1

If I understand this question correctly, the angle of reflection should be 90 degrees with respect to the mirror as well.  The angle of the incoming ray and the angle of the ray that is reflected will always be the same.

The ray that is coming in is called the indcident ray and the angle that it makes with an imaginary line perpendicular to the surface of the mirror is called the angle of incidence. 

Answer 2

The angle of reflecrion will be 90 degree as angle i = angle r