Which image illustrates diffraction

Figure C

Explanation:

Here different figure represents different situations

Figure A

it represents the situation of reflection of light in which light bounce back into the same medium after reflecting by a surface

Figure B

It show refraction of light in which light will bend from its path when light travels from one medium to other medium

Figure C)

here we can see that a plane wavefront incident on the small opening and after that it converts into spherical wavefront. So when light passes through a small opening then it gets diverge at the corners of the opening which is known as diffraction

Figure D

it is just the incident of light ray which get completely absorbed by the medium.