a
In going from left to right across a period in the periodic table, the basicity (i.e. proton affinity) decreases as the electronegativity of the atom possessing the lone pair of electrons increases. Hence basicity of \(\mathrm{NH}_{2}^-\) is higher than \(\mathrm{F}^-\). In moving down a group, as the atomic mass increases, basicity decreases. Hence \(F^{-}\) is more basic than \(\mathrm{I}^-\) and \(\mathrm{HO}^{-}\) is more basic than \(\mathrm{HS}^{-}\). Hence among the given ionic species, \(NH_2^-\) has maximum proton affinity.