Where does wave reflection occur?
- at boundaries between two different media
- while a wave is propagating through a single media
- when two waves moving in opposite directions collide
- none of the above

- at boundaries between two different media

Explanation:

Reflection is one phenomenon that occurs with waves. Reflection can occur when a wave meets the boundary between two different media: when this happens, the wave bounces back into the first media, but changing direction.

In fact, there is a precise relationship between the original direction of the wave and its final direction: calling "angle of incidence" the angle between the original direction and the normal to the surface, and "angle of reflection" the angle between the final direction and the normal to the surface, we have that

angle of reflection = angle of incidence

and this is called "law of reflection".