Question

The reason for greater range of oxidation states in actinoids is attributed to: A . 4f and 5d levels being close in energies. B . The radioactive nature of actinoids.C . Actinoid contraction. D . 5f, 6d and 7s levels having comparable energies.

Hint:
Actinides are F-block elements(atomic number 89 to103) with the general electronic configuration of outermost shell is $\left[ {{\text{Rn}}} \right]{\text{5}}{{\text{f}}^{{\text{1 - 14}}}}{\text{6}}{{\text{d}}^{{\text{0 - 1}}}}{\text{7}}{{\text{s}}^{\text{2}}}$. Where the last electron enters to the inner 5f-orbital of the actinides. Actinides are also known as rare earth metals.

Now according to the Aufbau principle(L+S value) the energy order of the orbitals should be $5f < 6d < 7s.$but due to the more diffuse orbitals their energy becomes more or less the same. As a result, electrons can be excited easily . Due to this reason actinides shows greater range of oxidation states. But if we consider lanthanides due to the comparatively small size of 4f orbital they have a limited number of oxidation states.