Answer: diffraction
Explanation:
De Broglie's hypothesis was that matters (including the electrons) had the same duality property of light: it behaves as a particles and as a wave.
One of the fundamental features of the waves is that they exhibit diffraction. So, when the electron was found to show diffraction, DeBroglie's hypothesis was verfied.
Diffraction is the bending of a wave around a barrier. When the wave passes through a small (narrow) opening (slit) it will generate patterns due to the interfence of the waves that experimented the diffraction.
