Which of the following ions is colourless in solution?

\(\mathrm{V}(23)=[\mathrm{Ar}] 3 \mathrm{~d}^{3}, 4 s^{2}\)
\(\mathrm{V}^{3+}=[\mathrm{Ar}] 3 d^{2}, 4 s^{0} \quad\) (two unpaired electrons)
\(
\begin{array}{l}
\operatorname{Cr}(24)=[\mathrm{Ar}] 3 d^{5}, 4 s^{1} \\
\mathrm{Cr}^{3+}=[\mathrm{Ar}] 3 d^{3}, 4 s^{0}
\end{array}
\)
(three unpaired electrons)
\(
\operatorname{Co}(27)=[\mathrm{Ar}] 3 d^{7}, 4 s^{2}
\)
\(
\mathrm{Co}^{2+}=[\mathrm{Ar}] 3 \mathrm{~d}^{7}, 4 s^{0}
\)
(three unpaired electrons)
\(
\mathrm{Sc}(21)=[\mathrm{Ar}] 3 d^{7}, 4 s^{2}
\)
\(\mathrm{Sc}^{3+}=[\mathrm{Ar}] 3 d^{0}, 4 s^{0} \quad\) (no unpaired electrons)
Thus, in \(\mathrm{Sc}^{3+}\) no unpaired \(d\) -electron is present. Hence, no \(d-d\) transition is possible and it is colourless ion.