The Aufbau Principle dictates that orbitals are filled in increasing order of energy, following the $(n+l)$ rule:

1. 3d: $n=3,l=2⟹n+l=5$
2. 4p: $n=4,l=1⟹n+l=5$

• When $(n+l)$ values are equal, the orbital with the lower value of $n$ has lower energy and is filled first.
• Therefore, $3d$ ($n=3$) is filled before $4p$ ($n=4$). Filling order: $\dots 4s\to 3d\to 4p\dots$