The nucleus of some isotopes will spontaneously undergo nuclear decay. these isotopes are said to be radioactive. what causes the nucleus of an isotope to be radioactive?
a. too many electrons in its orbitals
b. exposure to toxic chemicals
c. the net charge of the atom
d. the attraction of protons to electrons
e. an unstable ratio of protons and neutrons

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.