X-rays are produced by:

Explanation

X-ray photons are produced when electrons in the outermost shells of an atom drop to the inner shells. Hydrogen atoms do not emit X-rays because the electron energy levels are too closely spaced together to permit the emission of high-frequency radiation. Transitions of this kind are normally forbidden because the lower states are already filled. However, if an inner shell has a vacancy (an inner electron is missing, perhaps from being knocked away by a high-speed electron), an electron from one of the outer shells can drop in energy to fill the vacancy. Out of the given choices, only copper emits X-rays and is also used in X-ray tubes. Hence, it is the answer.