Answer: Option (A) is the correct answer.
Explanation:
An isotope with an unstable nucleus which decomposes readily and results in the emission of radiation and a nuclear electron or helium nucleus achieves a stable nuclear composition is known as a radioactive isotope.
For example, [tex]^{210}_{84}Po \rightarrow ^{4}_{2}He + ^{206}_{82}Pb[/tex]
Here isotope of polonium decomposes in order to achieve a stable nuclear composition.
Thus, we can conclude that an atom is unstable best describes a radioactive isotope.
