Reflection can be defined as a phenomenon which typically involves a change in the direction of a ray of light or waves as it falls on a smooth surface.
Simply stated, reflection occurs when a ray of light or wavefronts bounces off a smooth surface.
Hence, from the diagram provided above, the thin beam of light which illustrates reflection is the fourth because as it hit the surface, it bounced back to the medium from which it was originally propagated with.
