Final answer:
The correct statement is 'a. The 3d sub level is filled before the 4s sub level' as the 4s sublevel is at a lower energy than the 3d and fills first after the 3p sublevel.
Step-by-step explanation:
The statement that is correct is 'a. The 3d sub level is filled before the 4s sub level'. This might seem counterintuitive since 4s is at a lower energy level than 3d, and according to the Aufbau principle, electrons fill the lowest energy orbitals available first. As a result, after the 3p sublevel is filled, the 4s sublevel is filled next. Therefore, for example, potassium (K) has the electron configuration 1s²2s²2p¶3s²3p¶4s¹, indicating that after the 3p sublevel is full, the next electron enters the 4s sublevel. Furthermore, the third principal energy level can accommodate more than 8 electrons when we consider the addition of the five 3d orbitals. As for the order of orbital filling, it is not strictly numerical, as can be seen when the 4s is filled before the 3d, and the 5p is filled after the 4d. Also, the principal energy levels do not get closer together as they get further from the nucleus; rather, the energy levels get closer, which is why there is overlap in the filling of the sublevels.