Which one of the following ionic species has the greatest proton affinity to form stable compound?

Proton affinity is the tendency of a species to accept a proton (${\text{H}}^{+}$). It is related to the basic strength.Across a period, basicity decreases as electronegativity increases (${\text{NH}}_2^{-}>{\text{OH}}^{-}>{\text{F}}^{-}$).Down a group, basicity decreases as the size of the atom increases (${\text{F}}^{-}>{\text{Cl}}^{-}>{\text{Br}}^{-}>{\text{I}}^{-}$ and ${\text{OH}}^{-}>{\text{HS}}^{-}$).Since Nitrogen is less electronegative than Fluorine and smaller than Sulphur/Iodine, ${\text{NH}}_2^{-}$ is the strongest base and has the greatest proton affinity.