Concept:The spreading of waves when they pass through a narrow opening is called diffraction.Explanation:Plane waves have straight wavefronts travelling in one direction.When these waves pass through a narrow gap, only a small part of the wave gets through.The narrow gap behaves like a new wave source.The waves then spread out in all directions, forming circular wavefronts.This bending and spreading of waves after passing through a gap is known as diffraction.Answer:D. diffraction