## Chemistry and Chemical Reactivity (9th Edition)

The correct answer is Letter (d) Fe$^{2+}$
Fe$^{3+}$, Cr$^{+}$ and Mn$^{2+}$ all have an electron configuration of [Ar]$3d^{4}$ While $Fe^{2+}$ has an electron configuration of [Ar]$3d^{5}$ Therefore (a), (b) and (c) are incorrect leaving (d) as the correct answer