What force keeps stable nuclei permanently

together?

Electromagnetic force

Explanation:

It is stronger on the outside and protons repel each other

Nuclear force keeps the nuclei of a stable element together permanently.

Explanation:

Nucleus of any atom contains proton and neutron that is held together by the strong nuclear force between them. The force is very strong that it can hold the quarks that forms neutron and proton and binds it properly to form stable nuclei. The nuclear force inside the nucleus is way more powerful than the electromagnetic force, hence prevents in falling the atom's nucleus apart.