b
\((b)\) Increasing stability order :

\((a)\, [Cu(NH_3)_4]^{2+} <  [Cu(en)_2]^{2+} <  [Cu(trien)]^{2+}\)

Their formation entropy increases in the same order, because denticity of   ligand increases

\((b)\, [Fe(H_2O)_6]^{3+} <  [Fe(NO_2)_6]^{3-} < [Fe(NH_3)_6]^{3+}\)

\(NO_2^-\) is stronger ligand than \(NH_3\)

\((c)\, [Co(H_2O)_6]^{3+} < [Rh(H_2O)_6]^{3+} <  [Ir(H_2O)_6]^{3+}\)

\(Z_{eff}\) value increases from \(Co^{3+}\) to \(Ir^{3+}\)

\((d)\, [Cr(NH_3)_6]^{1+} < [Cr(NH_3)_6]^{2+} <  [Cr(NH_3)_6]^{3+}\)

Oxidation state of \(Cr\) atom increases from \(+ 1\) to \(+3\).