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.