c
\((d)\) In \(K_3[Cu(CN)_4]\)  \(Cu\) is in \(+1\) oxidation state hence has no unpaired electron hence colourless and diamagnetic.

\((b)\) In \({(N{H_4})_2}[TiC{l_6}]\,Ti\) is in \(+4\) oxidation state, hence has no unpaired electron hence colourless and diamagnetic.

\((c)\) In \(VOSO_4\), is in \(+4\) oxidation state, hence has no unpaired electron ,thus it is coloured and paramagnetic.

\((a)\) In \(K_2Cr_2O_7\)  \(Cr\) is in \(+6\) oxidation state, hence has no unpaired electron and thus it is diamagnetic. Though \(K_2Cr_2O_7\) has no unpaired electron but it is coloured. this is due to charge transfer.