Answer;
NaO-
NaO- is the element combination which is the most least likely because the ionization energy of sodium and oxygen are high.
Explanation;
Ionization energy is the amount of energy required to remove or dislodge the valence electrons from the outermost energy level. Ionization energy may be named as first or second and so forth depending on the electron removed from the outermost energy level.
The ease of element combination depends on the ionization energy of each atom of a given element. The higher the ionization energy of the atoms involved the less the ease of combining.