Final answer:
The 3d subshell has energy between the 4s and 4p subshells. After the 4s is filled, electrons fill the 3d subshell before moving on to the 4p subshell.
Step-by-step explanation:
The subshell that has energy between the 4s and 4p subshells is the 3d subshell. When discussing electron configurations and the order in which electrons fill subshells, it is observed that the 4s subshell is filled before the 3d subshell due to it having a slightly lower energy. However, after the 3d subshell is filled, which can hold up to 10 electrons, the next electrons occupy the higher energy 4p subshell. This energy overlap occurs as the atom's size increases, which affects the order of filling of the atomic sublevels.
In summary, as atomic numbers increase, electrons fill the 4s subshell, then the 3d subshell, and then the 4p subshell. This progression reflects the relative energy levels between these subshells where the 3d subshell fits in between the 4s and 4p subshells in terms of energy and their position on the periodic table.