The hardest photon coming out from a Coolidge tube belongs to:

Explanation

Characteristic X-rays are a unique property of individual elements.A high-energy electron knocks out an already existing electron in one of the shells of an anode atom, thus creating a vacancy in that particular shell of the atom. This event energizes the atom and an electron from a higher shell jumps to fill the vacancy in the lower shell so that the overall energy of the atom is decreased. This decrease in energy gives birth to the X-ray photon. So, characteristic X-ray photons are consequences of energy level transitions in the anode atoms. Continuous X-rays are created when free-moving electrons electromagnetically interact with nuclei. Continuous X-rays come from bremsstrahlung ( braking radiation) as the high-energy electrons approach the nucleus of a heavy metal. Since the electron could come very close to the nucleus (and give up a lot of energy to the production of an X-ray photon) or pass far from the nucleus (and give up relatively little energy), the range of the photon energies will be less and continuous.