Answer: (3) gamma radiation
Explanation:
An isotope can decay in 4 process:
1.) Alpha decay: In this process, alpha particles is emitted when a heavier nuclei decays into lighter nuclei. The alpha particle released has a charge of +2 units.
[tex]_Z^A\textrm{X}\rightarrow _{Z-2}^{A-4}+_2^4\alpha[/tex]
2.)Beta-decay: In this process, a neutron gets converted into a proton and an electron releasing a beta-particle. The beta particle released carries a charge of -1 units.
[tex]_Z^A\textrm{X}\rightarrow _{Z+1}^A\textrm{Y}+_{-1}^0\beta[/tex]
3.) Gamma ray emission: in this process, an unstable nuclei gives off excess energy by a spontaneous electromagnetic process and releases [tex]\gamma -radiations[/tex]. These radiations does not carry any charge and are electrically neutral.
[tex]_Z^A\textrm{X}^*\rightarrow _Z^A\textrm{X}+_0^0\gamma[/tex]
4.) Positron decay: In this process, a proton gets converted to neutron and an electron neutrino and releases positron particles. This particle carries a charge of +1 units.
[tex]_Z^A\textrm{X}\rightarrow _{Z-1}^A\textrm{Y}+_{+1}^0e[/tex]
In the question, it is given that when an isotope decays, it emits only energy. It is released when an isotope undergoes gamma decay and therefore, the correct answer is gamma radiation.