What substance would you add to nahco3(aq) to form a buffer solution?

Buffer solution is made by mixing a weak acid with its conjugate base or weak base with its conjugate acid. NaHCO₃(aq) is a conjugate salt and also a conjugate base of H₂CO₃(aq) acid. Hence H₂CO₃(aq) should be added to make the buffer solution.

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.