When wave-particle duality was applied to the electron, it explained why the energy of the electron is quantized because:

a. The electron is a standing wave that can only have an integer number of wavelengths
b. An electron can only exhibit specific velocities
c. The electron must have a specific mass
d. An electron will emit and absorb light quanta

a. The electron is a standing wave that can only have an integer number of wavelengths.

Explanation:

As per quantum physics, the theory of wave-particle duality refers to the notion that matter and light show the characteristics of both waves and particles, based on the case of the experiment. Much like light, the matter appeared to possess both wave and particle properties.  Large objects display very low wavelengths, but for small particles, the wavelength may be detected and important, as shown by a double-slit experiment with electrons.

8 carbon atoms forming single bonds with 18 hydrogen atoms; a triple bond exists between the third and fourth carbon atom from the right

explanation: