Which of the following ions will exhibit colour in aqueous...

Key Idea:- Colour is obtained as a consequence of $d-d($ or $f-f)$ transition, and for $d-d$ (or $f-f)$ transition, presence of unpaired electrons is the necessary condition. $\begin{gathered}\mathrm{La}^{3+}(\mathrm{Z}=57)=[\mathrm{Xe}]4 \mathrm{f}^0 5 \mathrm{ d}^0 6 \mathrm{ s}^0 \\ \text{ (no unpaired electron) }\\ \mathrm{Ti}^{3+}(\mathrm{Z}=22)=[\mathrm{Ar}]3 \mathrm{ d}^1 4 \mathrm{ s}^0 \\ \text{ (one unpaired electron) }\\ \mathrm{Lu}^{3+}(\mathrm{Z}=71)=[\mathrm{Xe}]4 \mathrm{f}^{14} 5 \mathrm{ d}^0 6 \mathrm{ s}^0 \\ \text{ (no unpaired electron) }\\ \mathrm{Sc}^{3+}(\mathrm{Z}=21)=[\mathrm{Ar}]3 \mathrm{ d}^0 4 \mathrm{ s}^0 \end{gathered} (no unpaired electron) Hence, only \mathrm{Ti}^{3+}$ will exhibit colour in aqueous solution.