Which compound is coloured due to charge transfer spectra and not due to d–d transitions?

All of these

The answer is (4), all of these.

\(K_2Cr_2O_7\) , \(KMnO_4\), and \(CrO_3\) are all coloured due to charge transfer spectra. In these compounds, the metal ion is in a high oxidation state and has a low-lying empty d orbital. The oxygen ligand can donate an electron to the metal ion, filling the empty d orbital. This produces a charge transfer complex, which is coloured.

d-d transitions occur when an electron in a d orbital of a metal ion is excited to a higher energy d orbital. This type of transition can also produce color, but it is not the dominant factor in the color of \(K_2Cr_2O_7\), \(KMnO_4\), and \(CrO_3\).