The correct answer is A. Unstable nucleus.
Radioactivity is the process by which an unstable atomic nucleus loses energy by radiation. This radiation can take the form of alpha particles, beta particles, or gamma rays. The type of radiation emitted depends on the type of atom that is radioactive.
Radioactivity is caused by the instability of the nucleus. The nucleus is made up of protons and neutrons, and these particles are held together by the strong nuclear force. However, the strong nuclear force is not strong enough to hold all nuclei together. Some nuclei are simply too unstable and will eventually decay, or break apart.
When a nucleus decays, it releases energy in the form of radiation. This radiation can damage cells and tissues, and it can even cause cancer. However, radioactivity can also be used for beneficial purposes, such as in medical imaging and nuclear power generation.
Option B is incorrect because a stable nucleus does not emit radiation.
Option C is incorrect because electronic configuration is the arrangement of electrons in an atom. It does not affect the stability of the nucleus.
Option D is incorrect because a stable electronic configuration does not affect the stability of the nucleus.