What is the term for the process that occurs when an atom of an element spontaneously breaks apart to create an atom of a different element and the release of energy as radiation or particles?
a. atomic deterioration
b. elemental deployment
c. nuclear degeneration
d. radioactive decay

Atoms of elements below atomic number 20 do not split because of stable nucleus.
Stability of nucleus is determined by neutron/proton ratio.
For light stable nuclides, number of protons ans neutrons are same. For heavy nuclides, number of neutrons is more than the number of protons.

Here the type of decay is alpha decay because number of nucleons(protons and neutrons) changes.
Hence option d is correct.