The electron first occupies the following orbital according to the Aufbau principle
Show Hint
Remember the Aufbau filling order:
\[
1s \rightarrow 2s \rightarrow 2p \rightarrow 3s \rightarrow 3p \rightarrow 4s \rightarrow 3d \rightarrow 4p \rightarrow 5s.
\]
The \(4s\) orbital is filled before the \(3d\) orbital.