Which one has one unpaired electron in the valence shell?

Correct Answer: D. Fe-1

The ion Fe-1 represents iron (Fe) with a charge of -1. In this ion, iron has gained an extra electron, resulting in a negatively charged species.
The electron configuration of a neutral iron atom (Fe) is [Ar] 3d6 4s2, which means it has six electrons in its outermost 3d subshell. However, when iron gains an electron to become Fe-1, its electron configuration becomes [Ar] 3d7. In this configuration, there are seven electrons in the 3d subshell.
The important characteristic of Fe-1 is that it has one unpaired electron in its 3d subshell. Unpaired electrons are significant in chemical reactions, as they can participate in bonding and interactions with other atoms or ions. In the case of Fe-1, this unpaired electron can make it reactive and capable of forming chemical bonds with other species to achieve a more stable electron configuration.