Which of the following is a possible application for discoveries made during ocean exploration?

The answer is all of the above.     I got it correct

And a) oxygen i believe

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.