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.

the electrical  conductivity  of asolution  of an electrolyte is measured by determining the resistance of thesolution  between two flat or cylindrical electrodes separated by a fixed distance.

explanation:

let the different atoms be x and y. each with four protons and same mass will be represented as follows.

mass number is number of protons + number of neutrons.

            and  

since, it is given that one has a negative charge while the other is neutral. therefore, there will be increase in number of protons and the atom will be represented as follows.

according to periodic table, mass number of beryllium is 9 and it has atomic number 4 so, the atoms will be and .  

the protons and neutrons will be located inside the nucleus whereas the electrons will be revolving around the nucleus.