The answer is: e. an unstable ratio of protons and neutrons.
Neutrons stabilize the nucleus, because they attract protons and reduce electrical repulsion between protons.
The larger is neutron-proton ratio, the higher is probability for nuclear decay.
Ratio of protons to neutrons in a stable atom is 1 : 1.
For example, nitrogen atom (N-14) has 7 protons (p⁺) and 7 neutrons (n°). The neutron–proton ratio (N/Z ratio or nuclear ratio) is 7 : 7 ( 1 : 1 ).
Another example, neon atom (Ne-20) has 10 protons (p⁺) and 10 neutrons (n°). The neutron–proton ratio is 10 : 10 ( 1 : 1 ).
The neutron–proton ratio generally increases with increasing atomic number.
