The correct answer is radioactive.
Atoms are composed of three kinds of sub-atomic particles, that is, the neutrons and protons in the nucleus and the electrons orbiting the nucleus. Some of the substances are radioactive as the nucleus of each atom is unstable and emits nuclear radiation in the form of beta particles, alpha particles, or gamma rays.
In the radioactive atoms, the binding energy is not powerful enough to hold the nucleus in combination, and the nuclei of these atoms are considered to be unstable. The unstable atoms will lose protons and neutrons as they try to become stable.