4f subshell of lanthanides is well shielded by \(5 \mathrm{d}\) and \(6 \mathrm{s}\) orbitals so electrons of \(4 \mathrm{f}\) subshell don't participate in bonding.
Radius of lanthanides gradually decreases as atomic number increses due to poor shielding effect od f subshell.
Lanthanides usually show +3 oxidation state. So, answer is D.