What is the angle of incidence of a ray if the reflected ray is at an angle of 90° to the incident ray?

By the laws of reflection, angle of incidence (i) is equal to the angle of reflection (r).
i.e. i = r (i)
Also, they are on the opposite sides of the normal. Since, they are in the same plane, the angle between them θ = i + r. (ii)
From (i) and (ii), θ = 2i
It is given θ = 90° ⇒ i = 45°