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.

out of these options:

ability to burn paper

pure sodium reacting violently with water

the boiling point of water

helium does not tend to react with anything

i would say this is a physical property:

helium does not tend to react with anything

why? :

i said helium does not tend to react with anything because well, it reacts to ! but ability to burn paper, pure sodium reacting violently with water, & the boiling point of water are chemical or

physical properties: the properties of a substance that can be observed without changing the chemical makeup of the substance

chemical properties: properties of a substance relating to the chemical nature and reactivity of the substance