Because of lanthanoid contraction, which of the following pairs of...

Zr$\left(40\right)$ and $Hf\left(72\right)$: Because of the lanthanoid contraction, $Zr$ (atomic radii $160$ pm)$and Hf$ (atomic radii $158$ pm) have nearly the same atomic radii.

Lanthanoids include the elements from lanthanum $La\left(Z = 57\right) to lutetium$Lu \left(Z = 71\right)$. Zirconium$Zr\left(40\right) belong$to$the second transition series \left(4d\right) and Hf\left(72\right) belongs to the third transition series \left(5d\right). Lanthanoid contraction is related to the intervention of the$4f -$orbitals, which are filled before the $5d -$series of elements start. The filling of $4f$, if orbitals before 5d -orbitals result in a regular decrease in atomic radii, which compensates the expected increases in atomic size with increasing atomic number. As a result of lanthanoid contraction, the elements of second and third transition series have almost similar atomic radii.