The diamagnetic ion is

Electronic Configurations of ions given are as 

  $$\begin{gathered} 21{\mathrm{Sc}}^{3+};3d^{\circ }4S^{\circ } \\ \begin{array}{r}\mathit{22}{\mathit{Ti}}^{\mathit{3}\mathit{+}}\mathit{;}\mathit{3}{d}^{\mathit{1}}\mathit{4}{S}^{\mathit{\circ }}\\ \mathit{23}{V}^{\mathit{2}\mathit{+}}\mathit{;}\mathit{3}{d}^{\mathit{3}}\mathit{4}{S}^{\mathit{\circ }}\\ \mathit{ }\mathit{ }\mathit{ }\mathit{ }\mathit{ }\mathit{24}{\mathit{Cr}}^{\mathit{2}\mathit{+}}\mathit{;}\mathit{3}{d}^{\mathit{4}}\mathit{4}{S}^{\mathit{\circ }}\end{array} \end{gathered}$$

In ${\mathrm{Sc}}^{3+}$ all electrons are paired and hence it diamagnetic