The 4f-subshell is successively filled for

The 4f level is successively filled up in lanthanides also called rare earths as the general configaration of lanthanides is $(\mathrm{n}-2){\mathrm{f}}^{1-14}(\mathrm{n}-1){\mathrm{d}}^{0-1} \mathrm{ns}{}^0\text{ or }1\text{ or }2$.

Therefore, option A is the correct answer.