C

To determine the energy of characteristic X-rays, we refer to the energy levels of the electrons in the atom. Characteristic X-rays are produced when an electron transitions from a higher energy level to a lower one, emitting a photon with energy equal to the difference in energy between the two levels. If the characteristic energy is defined to be around 40 keV, the emitted X-ray photon would have energy equal to that value. Since the question specifically mentions 'equal to 40 keV', the correct answer is Option C.