I think the anwer is electrolyte :)... i had it on a test a couple days ago.
The word that best fits is frequency. The sentence is The frequency of a wave is a measure of the amount of energy it carries. There is a direct relationship between frequency and energy of a wave. The constant of proportionality is h, Planck's constant. So, Energy = h * frequency. So, the answer is frequency.<span> Although you should know that wavelength is related to frequency and so it is also a measure of the amount of energy and it should also be other valid answer.</span>
According to the Aufbau principle, , electrons orbiting one or more atoms fill the lowest available energy levels before filling higher levels (e.g., 1s before 2s).