\(\mathrm{Zn}\) atomic radii is \(137 \mathrm{pm}\)
\(\mathrm{Fe}\) atomic radii is \(126 \mathrm{pm}\)
\(\mathrm{Fe}^{3+}\) ionic radius is \(63 \mathrm{pm}\),
\(\mathrm{Fe}^{2+}\) has an ionic radius of \(77 \mathrm{pm}\).
Thus, the correct order of their size is
\(\mathrm{Zn} > \mathrm{Fe} > \mathrm{Fe}^{2+} > \mathrm{Fe}^{3+}\)
Hence, the correct option is (a).