Answer
D. the bending of light as it passes through a small opening
Step-by-step explanation
Diffraction is the bending of light around obstacles and openings. The amount of diffraction increases with increasing wavelength.
Light is also a wave, so if it goes through a narrow slit or small hole, it changes from a plane wave to a spherical one. This transformation is the same as with waves of water. Diffraction describes that change in light's direction.
Therefore, the correct answer is:
D. the bending of light as it passes through a small opening