Which of these is most likely a characteristic of a parent isotope before it releases radioactive particles? select one: a. stable nucleus b. stable electrons c. unstable electrons d. unstable nucleus

The correct option is D.
Isotopic substances typically have unstable nuclei and that is why they disintegrate in order to become stable. During the process of disintegration, the nucleus of the unstable substance will be split into two smaller fragments, which are more stable that the parent isotope. This disintegration may also be accompanied by radiations.