Define "ionizing radiation".

The definition of "ionizing radiation" focuses on its ability to interact with matter at the atomic level. Specifically, ionizing radiation is characterized by carrying enough energy to dislodge electrons from atoms, resulting in the formation of ions. This process can lead to chemical changes in the materials it encounters, which is why ionizing radiation is particularly important in fields such as nuclear medicine, radiation therapy, and radiation safety.

The other choices do not accurately reflect this fundamental characteristic of ionizing radiation. While it's true that some forms of radiation move at the speed of light, this aspect does not define ionizing radiation itself. Furthermore, while ionizing radiation can affect living organisms, it is not limited to them; it can interact with all types of matter. Lastly, the ability to penetrate solid objects varies among different types of radiation, and this factor does not encapsulate the essence of ionizing radiation. Thus, the correct answer highlighting the energy of ionizing radiation to liberate electrons captures the critical understanding necessary for anyone working with or studying radiation-related topics.