Answer
The complete nuclear reaction is
[tex]_{12}^{26}Mg\rightarrow\text{ }_{13}^{26}Mg+\text{ }_{-1}^0e[/tex]
Explanation
The incomplete equation is that of beta radiation; where the Mg emits an electron. Beta radiation is the emission of electrons or positrons, alpha radiation is the name for the emission of an alpha particle in fact an helium nuclei, and gamma radiation is the term used for the emission of energetic photons.
The complete nuclear reaction is shown below:
[tex]_{12}^{26}Mg\rightarrow\text{ }_{13}^{26}Mg\text{ }+\text{ }_{-1}^0e[/tex]
