The correct answer to the question is reflection.
EXPLANATION:
Before coming into any conclusion, first we have to understand reflection.
Reflection is the type of phenomenon in which a wave is reflected back to the same medium when it incidents on a obstacle. The obstacle may be any opaque body, mirror etc.
As per the diagram, the wave incidents obliquely on the wall.
After incident on the wall, the wave is reflected back to the same medium.Here, the wall acts as a obstacle. This property of wave is called as reflection.
Diffraction and reflection can not be the suitable answer for it . It is so because diffraction refers to the bending nature of light at the edge of an obstacle while refraction is the optical phenomenon in which the light ray will be bent towards or away from the normal at the interface of the refracting surface.
Hence, the correct answer to the question is reflection.
