Correct Answer - 3
The atomic radii of the second and third transition series are almost the same . This phenomenon is associated with the intervention of the `4f` orbitals which must be filled before the `5d` series of elements begin. The filling of `4f before 5d` orbital results in a regular decrease in atomic radii called Lanthanoid contraction which essentially compensates for the expected increase in atomic size with increasing atomic number . The net result of the lanthanoid contraction is that second and third d series exhibit similar radii `(e.g.,Zr 160)` pm, `Hf 159 pm)`
