Among the following, sub shells which have to be filled before filling ‘3d’ orbital

In the atomic orbitals, electrons are filling up in the increasing order Of their energies ( n +l  ). This means the orbital with least energy is filled first with the electrons.

For $2\mathrm{s}\to \mathrm{n}+1=2+0=2;\mathrm{For} 3\mathrm{s}\to \mathrm{n}+\mathrm{l}=3+0=3$

For  $3\mathrm{p}\to \mathrm{n}+\mathrm{l}=3+1=4; \mathrm{For} 4\mathrm{s}\to \mathrm{n}+\mathrm{l}=4+0=4$

$\mathrm{For} 3\mathrm{d}\to \mathrm{n}+1=3+2=5$

Hence, the given all orbitals must be filled before filling 3d orbital