How much energy is required to remove the most loosely bound electron from a neutral atom of carbon in the gaseous phase?

This is called the first ionization energy, and its value is 1086 kj/mol 

The problem here is that according to aufbau principle before 3d orbital 4s subshell comes.after filling 3p orbital with electrons you have 4 electrons left, you place two in 4s with opposite spin and the other two in 3d in any of its two orbitals of total five orbitals with the same spin.