Atomic structure — practice question

For which pair does every species have the same number of unpaired electrons?

• A$\text{Al}$ and $\text{Cu}^{2+}$
• B$\text{Ca}$ and $\text{Cr}^{3+}$
• C$\text{Ca}$ and $\text{Ni}^{2+}$
• D$\text{Fe}^{3+}$ and $\text{O}^{2-}$