Explanation

(b)
These are isoelectronic species and their radii decreases with increasing their atomic number due to increasing effective nuclear charge ($Z_{eff})$
$\left( Z_{eff} \right) = Z - \sigma$
where, $Z_{eff}$=effective nuclear charge, $Z$=atomic number and $\sigma =$screening constant. For $F^{-},O^{2 -}\ and\ N^{3 -}$, the value of $\sigma$ is constant due to equal number of electrons. So, order of $Z_{eff}$ is $F^{-} < O^{2 -} > N^{3 -}$
hence, order of radii
=$\ F^{-} < O^{2 -} < N^{3 -}\left( radii\ \propto \frac{1}{Z_{eff}} \right).$