Final answer:
In the ground state of atoms, the filling of 3d sublevels begins with scandium (Sc) and ends with copper (Cu), making the correct answer 4. SC and ending with Cu.
Step-by-step explanation:
The question asks about the process of electron filling in the 3d sublevels within transition metals. In the ground state of atoms, this process begins with scandium (Sc) and ends with copper (Cu). So the correct answer to the student's question is 4. Sc and ending with Cu. Scandium, with atomic number 21, is where the 3d subshell begins to fill, and it continues to fill until the 3d subshell is complete with copper (Cu). Following this, electrons will then fill the 4p subshell.
The provided materials clarify that the 3d subshell is capable of holding up to 10 electrons, which correlates with there being 5 distinct d orbitals. Moreover, the principles of electron configuration suggest that half-filled and fully filled subshells, such as those observed in chromium (Cr) and copper (Cu), have enhanced stability due to electron distribution.
The unusual filling order of electron subshells, particularly after the 3p subshell is complete, leads to the 4s sublevel filling before the 3d sublevel. This is why the 3d sublevel does not start filling immediately after Argon (Ar), but rather after the 4s is filled, beginning with scandium (Sc).