Chapter 7 The Structure of Atoms and Periodic Trends - 7-4 Electron Configurations of Ions - Review & Check for Section 7-4 - Page 266: 1

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