Answer:
D) The nuclei of unstable isotopes
Explanation:
For example, the carbon-14 in carbon dating is radioactive.
When it decays, a neutron in the nucleus is converted into a proton, an electron, and an antineutrino.
[tex]_{0}^{1}\text{n} \longrightarrow \, _{1}^{1}\text{p} + \, _{-1}^{0}\text{e} + \overline{\nu}_{\text{e}}[/tex]
