b
\(\mathrm{Sc} \rightarrow[\mathrm{Ar}] 3 d^{1} 4 s^{2}, \mathrm{Sc}^{3+} \rightarrow[\mathrm{Ar}]\)

\(\mathrm{Ti} \rightarrow[\mathrm{Ar}] 3 d^{2} 4 s^{2}, \mathrm{Ti}^{3+} \rightarrow[\mathrm{Ar}] 3 d^{1}\)

\(\mathrm{Ni} \rightarrow[\mathrm{Ar}] 3 d^{8} 4 s^{2}, \mathrm{Ni}^{2+} \rightarrow[\mathrm{Ar}] 3 d^{8}\)

\(\mathrm{Cu} \rightarrow[\mathrm{Ar}] 3 d^{10} 4 s^{1}, \mathrm{Cu}^{+} \rightarrow[\mathrm{Ar}] 3 d^{10}\)

\(\mathrm{Co} \rightarrow[\mathrm{Ar}] 3 d^{7} 4 s^{2}, \mathrm{Co}^{2+} \rightarrow[\mathrm{Ar}] 3 d^{2}\)

\(\mathrm{Ti}^{3+}, \mathrm{Ni}^{2+}\) and \(\mathrm{Co}^{2+}\) are coloured due to presence of unpaired electrons.