In the order of electron configuration, which sublevel is filled before the 4d sublevel?

The 4d sublevel is filled after the 5s sublevel due to the principles of the Aufbau principle, which states that electrons fill atomic orbitals starting from the lowest energy level to the highest. In the context of the periodic table and electron configuration, the 5s sublevel has a lower energy than the 4d sublevel, meaning that it will be filled first as one moves through the periodic table.

The filling of these sublevels is determined by the specific order dictated by both their principal quantum number and angular momentum quantum number. In general, the 4d sublevel (with a principal quantum number of 4 and an angular momentum of 2) will be filled after 5s (with a principal quantum number of 5 and an angular momentum of 0). Thus, before electrons are added to the 4d sublevel, the 5s sublevel must be completely filled.

Other choices, like 5p, 4p, and 5s, are less relevant because they pertain either to higher energy levels that come after 5s (such as 5p) or to the same principal quantum number level but have different energy arrangements (4p). Therefore, the correct answer