Answer
The particle nature. The wave nature.
Work Step by Step
The photoelectric effect, as discussed on page 587, provides conclusive evidence that light and electrons have a particle nature.
The discreteness of Bohr energy levels, as explained on pages 608-609, is best explained by the wave nature of electrons. According to the de Broglie model, the circumference of the electron’s orbit equals a whole number of electron wavelengths.
