What is radioactivity?

Hint: An unstable nucleus tends to decompose spontaneously, or decay, into a more stable configuration but do so in a few specific ways, either by emitting certain particles or certain forms of electromagnetic energy. Radioactive decay is a property exhibited by several naturally occurring elements as well as by artificially produced isotopes of the elements.

Complete step by step answer:
Radioactivity is the property being exhibited by certain types of matter in which they emit energy and subatomic particles spontaneously and simultaneously. Radioactivity is basically a phenomenon shown by individual atomic nuclei. A sample material containing radioactive nuclei is considered as radioactive.
Radioactive decay is described as the process by which an unstable atomic nucleus loses energy by radiation. The process by which an unstable atomic nucleus loses energy by radiation is called radioactive decay. The radioactive decays per unit time are directly proportional to the number of nuclei of radioactive compounds in the sample.
The emissions of the most common types of spontaneous radioactive decay are:
The alpha $\left( \alpha \right)$ particle
The beta $\left( \beta \right)$ particle
The gamma $\left( \gamma \right)$ ray
The neutrino
The alpha particle is actually the nucleus of a Helium-4 atom having two positive charges, protons, and two neutral particles, neutrons. . Such charged atoms are called ions. The neutral helium atom has two electrons revolving outside its nucleus balancing these two positive charges.
Beta particles may be negatively charged (beta minus, symbolized by ${{e}^{-}}$), or positively charged (beta plus, symbolized by ${{e}^{+}}$). The beta minus $\left[ {{\beta }^{-}} \right]$ particle is actually an electron created in the nucleus during beta decay without any alteration to the orbital electron cloud of the atom. The beta plus particle $\left[ {{\beta }^{+}} \right]$ , also known as the positron, is the antiparticle of the electron. When brought together, electrons and positrons will mutually annihilate each other.
Gamma rays are the electromagnetic radiations such as light, radio waves, and X-rays.
Beta radioactivity also produces the neutrino and the antineutrino. These are the particles that have no charge and very little mass.

So, the correct answer is “Option A”.

Note:
Radioactivity is the giving off of rays of particles or energy, such as electromagnetic waves, by the process of breaking apart atoms, or nuclei, of certain elements. The rays or particles that are given off when atoms break apart are called radioactivity. In radioactivity, the matter, subatomic particles, and energy are emitted spontaneously and simultaneously.