According to the aufbau principle, which orbital is filled immediately before each of the 4s, 3d, 4f and 5p?

 As per the Aufbau’s principle, the orbital or subshell with the lowest energy should be filled first. The ascending order of orbital’s energy is given by 1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, 5s, 4d, 5p,6s, 4f, 5d, 6p, 7s

Before 4s , 3p orbitals are filled

Before3d ,4s orbitals are filled

Before 4f,6s orbitals are filled

Before 5p , 4d orbitals are filled