b
\(\mathrm{Ti} \rightarrow[\mathrm{Ar}] 3 d^{2} 4 s^{2}, \mathrm{Ti}^{3+} \rightarrow[\mathrm{Ar}] 3 d^{1} 4 s^{0}\) \((1\) unpaired electron)

\(\mathrm{Cr} \rightarrow[\mathrm{Ar}] 3 d^{4} 4 s^{2}, \mathrm{Cr}^{3+} \rightarrow[\mathrm{Ar}] 3 d^{3} 4 s^{0}\) \((3\) unpaired electrons)

\(\mathrm{Co} \rightarrow[\mathrm{Ar}] 3 d^{7} 4 s^{2}, \mathrm{Co}^{3+} \rightarrow[\mathrm{Ar}] 3 d^{6} 4 s^{0}\) \((0 \text { unpaired electrons because of pairing })\)

\(\mathrm{Zn} \rightarrow[\mathrm{Ar}] 3 d^{10} 4 s^{2}, \mathrm{Zn}^{2+} \rightarrow[\mathrm{Ar}] 3 d^{10}\) (no unpaired electrons)

\(\left[\mathrm{Cr}\left(\mathrm{NH}_{3}\right)_{6}\right]^{3+}\) exhibits highest paramagnetic behaviour as it contains \(3\) unpaired electrons.