When does radioactive decay occur
a. when an atom gains electrons
b. when neutrons accumulate in the nucleus
c. when the nucleus decays
d. when an atom loses electrons

C.when the nucleus decays

Explanation:

In chemical reactions, the outermost electrons of atoms are re-arranged somehow without the involvement of the nuclei of the participating atoms of the elements.

Nuclear reactions causes changes within the atomic nucleus. For every atomic nucleus, a specific neutron/proton ratio ensures stability. When the stability ratio differs an atom becomes unstable and splits into one or more other nuclei with the emission of small particles of matter. This is what radioactivity entails.