Which orbital is filled immediately after the 4s orbital according to the Aufbau principle?

Explanation

Orbitals fill in order of increasing n plus l, and where that sum ties the lower n goes first. For 3d the sum is 5 and for 4p it is also 5, but 3d has the lower principal quantum number and so fills first, which is why the first transition series follows calcium. The 4p orbitals are filled only after the 3d subshell is complete at zinc.