After 3d-sub level is completely filled the differentiating electron enters into ....... sub level.

According to Aufbau principle electrons filled in the orbitals with the increasing order$\text{ of energy }(n+l)\text{. For }3\mathrm{d}\text{ orbital }n+l=3+2=5\text{. }$

$\text{ For }4\mathrm{s}\text{ orbital, }n+l=4+0=4$

$\text{ For }4\mathrm{p}\text{ orbital, }n+l=4+1=5$

$\text{ For }4\text{ forbital, }n+l=4+3=7$

$\text{ For }5\mathrm{s}\text{ orbital, }n+l=5+0=5$

Even though both 4p and 5s orbitals have same n+I value but the orbital which has the least ‘n’ value it has lowest energy with high stability.

Therefore, the electron enters into 4p sub shell after 3d sub shell is completely filled.