Which one of the following transition metal ions is diamagnetic?

Transition metal ion having absence of unpaired electron is diamagnetic. (1) $\mathrm{Co}^{2+}=[\mathrm{Ar}] 3 d^7$ (three unpaired electrons) (2) $\mathrm{Ni}^{2+}=[\mathrm{Ar}] 3 d^8$ (two unpaired electrons) (3) $\mathrm{Cu}^{2+}=[\mathrm{Ar}] 3 d^9$ (one unpaired electron) (4) $\mathrm{Zn}^{2+}=[\mathrm{Ar}] 3 d^{10}$ (no unpaired electron) Thus, $\mathrm{Zn}^{2+}$ is diamagnetic.