The Aufbau principle states that electrons are filled into atomic orbitals in the increasing order of orbital energy level. According to the Aufbau principle, the available atomic orbitals with the lowest energy levels are occupied before those with higher energy levels.
Electronic configuration of C, Cr, Fe, Cu is given below.
C (Z=6) = 1s2 2s2 2p2 or [He] 2s2 2p2
Cr (Z = 24) = [Ar] 3d5 4s1
Fe (Z = 26) = [Ar] 3d6 4s2
Cu (Z = 29) = [Ar] 3d10 4s1