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. 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.
