* Fission is a nuclear reaction where a heavy atomic nucleus splits into two or more lighter nuclei, releasing a tremendous amount of energy.

* Stability in an atom is determined by the balance between the strong nuclear force holding the nucleus together and the electrostatic repulsion between protons.

* Unstable atoms, like Uranium-235, have a nucleus where this balance is disrupted. They have an excess of neutrons, making the nucleus very prone to splitting when struck by a neutron.

Let's look at the other options:

* Energetic: While fission releases a lot of energy, that's the *result* of the reaction, not what makes it possible.

* Very small: The size of the atom itself isn't the primary factor driving fission.

* Stable: Stable atoms are the opposite of what's needed for fission. They are not likely to split.

In summary: The key to fission is the instability of certain atomic nuclei.