Which property ensures that the electrons balance the protons in an atom?

The property that ensures the balance between electrons and protons in an atom is the negative charge of electrons. In a neutral atom, the number of electrons is equal to the number of protons. Protons carry a positive charge, and consequently, the negatively charged electrons attract them, resulting in an electrically neutral atom. This fundamental characteristic of electrons is crucial because it defines the atom's chemical behavior, stability, and how it interacts with other atoms.

The other options do not directly relate to the balance of charge in an atom. The quantity of neutrons affects the atomic mass and isotopes but does not impact charge balance. Neutrons themselves are neutral and do not possess a charge to balance protons. Although stable atomic mass is important for the identity of an element, it does not influence the charge neutrality. Thus, the property that directly ensures electrons and protons balance in an atom is the negative charge of electrons.