$(b)$ 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.