Answer: F-
Explanation:
Monatomic anion is considering two terms that can give us the right answer to this question.
- Monatomic as the name says, has mono which means one, and atom. The meaning of it consist that something is made of one atom. In this case, NH4+ and SO4 2- can't be the right answers(they are polyatomic) but F- and Fr+ are considering one atom ions. Examples of more monatomic ions are H+ or Na+.
- Anion is considering negative ion. For example, OH- or CL- and because in the question we have 'anion' it can't be Fr+ so the correct answer is F-.