Which radiation is produced when electrons are knocked out of orbit?

Characteristic radiation is produced when an electron is knocked out of its orbit, typically during interactions in an X-ray tube. This process occurs when a high-energy photon interacts with an inner-shell electron of an atom, displacing it from its orbit. As this electron is ejected, the atom becomes ionized and unstable. To restore stability, an electron from a higher energy level falls to the lower energy level that was just vacated, and in doing so, it releases energy in the form of a photon. This photon has a specific energy that corresponds to the difference in energy levels of the electrons involved, which is why it is termed "characteristic" radiation; the emitted radiation is characteristic of the material's atomic structure.

In contrast, Bremsstrahlung radiation occurs when electrons are decelerated or deflected by the electric fields of atomic nuclei rather than being knocked out of orbit. X-ray radiation is a broader term that encompasses both characteristic and Bremsstrahlung radiation. Thermal radiation refers to the emission of electromagnetic radiation due to the temperature of a substance and is not produced through the ionization of electrons. Therefore, the correct choice directly identifies the type of radiation associated with the ionization process described in the question.