Which of the following can be caused by ionizing radiation?

Ionizing radiation has the capability to induce radioactivity in materials it interacts with. When ionizing radiation, such as gamma rays, X-rays, or particles like alpha and beta particles, strike an atom, they can remove electrons from the atoms, which may lead to the formation of unstable, radioactive isotopes. This process can alter the atomic structure and result in the emission of radiation from the affected substance.

In contrast, while heat generation may occur as a byproduct of certain types of radiation interaction, it is not a direct consequence of ionizing radiation itself. Increased tissue growth is more commonly associated with factors like hormonal changes or healing processes rather than the effects of radiation. Enhanced nutrient absorption does not relate to ionizing radiation; rather, this phenomenon is influenced by physiological factors like intestinal health or dietary composition.

Thus, the capacity of ionizing radiation to induce radioactivity highlights its significant impact on atomic stability and radioisotope formation, making it the correct answer in this context.