Radioactive material is defined as any material:

The definition of radioactive material primarily hinges on the characteristic of its atomic structure. Material classified as radioactive contains atoms that are unstable and, as a result, emit energy in the form of radiation. This radiation can be in the form of alpha particles, beta particles, or gamma rays. The emission of this energy distinguishes radioactive materials from non-radioactive materials, making option B the correct answer based on this definition.

In contrast, material found readily in nature does not encompass all scenarios of radioactivity since not all naturally occurring substances are radioactive. Similarly, sensitivity to radiation does not directly correlate with being radioactive; some materials unrelated to radioactivity can also be sensitive to radiation. Furthermore, material that has been exposed to gamma radiation may not necessarily indicate that it is radioactive unless it retains unstable isotopes after exposure. Thus, focusing on the atomic characteristics and energy emission makes option B the most accurate description of radioactive material.