Final answer:
The fluoride ion (F⁻) is the monatomic anion listed. Monatomic anions are named by adding an '-ide' suffix to the element name and are negatively charged due to the gain of electrons.
Step-by-step explanation:
The question asks which ion is a monatomic anion. A monatomic anion is an ion consisting of a single atom with a negative charge. Anions are formed when an atom gains electrons, achieving the electron configuration of a noble gas, which results in the atom having more electrons than protons. According to the provided reference, a monatomic anion is named by replacing the end of the element's name with '-ide' and then adding the word 'ion'.
For example, the fluoride ion is denoted as F⁻ and is formed when a fluorine atom gains one electron. Thus, based on the suffix and the charge of the ions listed in the question, the fluoride ion (F⁻) is the correct answer as it is the only monatomic anion given in the list. The other ions such as NH4+, Fr+, and SO4 2- are either cations or polyatomic ions.