A wave is incident on the surface of a mirror at an angle of 30° with the normal. At what angle will the outgoing wave reflect? 0° 30° 60° 90°

Hello!
As I understand, a "wave" means "electromagnetic wave," or "light." These waves may reflect from a mirror (other waves also may reflect, for example waves on a water surface may reflect from a wall).
The main law of reflection is that an angle of reflection is the same as an angle of incidence. They are both measured from the normal at the point of incidence (normal is a straight line perpendicular to a mirror at the point of their intersection). For a plain mirror these angles might be measured from a mirror's plane, and such angles are also equal. All normals to a spherical mirror, for example, go through the sphere's center.
So the answer is: the angle of reflection is 30 degrees with the normal (the same as for an incident wave).