An aqueous solution of sodium hydroxide reacts completely with a solution of hydrochloric acid. after all the water evaporates from the reaction mixture, what will remain

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.