Final answer:
The last electron in an iron atom occupies a D) 3d orbital.
Step-by-step explanation:
The last electron in an iron atom occupies a 3d orbital. Iron is written as [Ar]4s²3d⁾ and losing 2 electrons from the s-orbital (4s) or 2 s- and 1 d- or 1 s- and 1 d- orbital (3d&sup5;) electron are fairly stable oxidation states.