So we want to know what is the change in isotopes over time due to the emission of radioactive particles. The radioactive decay is the process of emiting particles from the nucleus of the atom. The consequence is that the amount of the radioactive material is loosing its mass over time by emmiting energy or particles. It can be alpha particles- those are helium nucleus, beta particles- those are electrons and it can be gamma particles- those are high energy photons. So the correct answer is D, Radioactive decay.