Final answer:
Fe²⁺ with a configuration of [Ar]3d⁶ is paramagnetic with four unpaired electrons.
Step-by-step explanation:
The electron configuration of Fe²⁺ is [Ar]3d⁶, which indicates the presence of unpaired electrons. According to Hund's rule, the electrons will fill the orbitals in a way that maximizes the number of unpaired electrons. As such, the 3d orbitals will have four unpaired electrons in the case of the Fe²⁺ ion. Therefore, Fe²⁺ is paramagnetic with four unpaired electrons, which aligns with the option (D) among the choices provided.