What material would be safest to use as an insulator to cover electrical wires? A:Rubber
B:Tin
C:Water
D:Aluminum

That would be rubber because rubber does not conduct electricity so A is the right answer

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.