Answer: Ratio of Protons to neutrons
Explanation: An atom consist of nucleus which contains the protons and neutrons and the ratio of the later two defines the stability of an atom.
Thus in other words, we can say that the stability of atom is determined by the nuclear stability of atom.
If the ration of the neutron to the proton is balanced, the atom is stabilized and if it is not balanced, it will lead to the radioactive decay of the atom.