When an atom of the unstable isotope Na-24 decays, it becomes an atom of Mg-24 because the Na-24 atom spontaneously releases:

(1) an alpha particle

(2) a beta particle

(3) a neutron

(4) a positron

(2) a beta particle, because and atom "moves up" one place in periodic table, and such thing happens when beta decay occurs.

A sodium-24 (Na-24) atom has atomic number 11 and mass 24, while a magnesium (Mg) atom has atomic number 12 and mass 24. Therefore the only change is that a particle with (-1) atomic number and (0) mass must have been released by Na-24 to get Mg-24.

This description fits a beta particle, which has a (-1) atomic number and (0) mass.