Which of the Following Is the Best Definition of Radioactivity

Which of the following is the best definition of radioactivity?

A) The logarithmic relationship between the logarithm of the activity coefficient and the radiation emitted by a radioactive nucleus.
B) The relative activity of the nuclear properties of electromagnetic radiation in the radio wave frequencies of the electromagnetic spectrum.
C) The spontaneous emission of particles and/or electromagnetic radiation resulting from the decay of an atomic nucleus.
D) The emission of electrons from an ionic compound that has unbalanced charge, after being stimulated by the radiation from the radio wave frequencies of the electromagnetic spectrum.
E) The unnatural radiation emitted by manmade nuclear processes such as nuclear power plants and atomic bombs.