Aromatic diazonium salts are more stable than aliphatic diazonium salts. The higher stability of aryl diazonium salts is due to resonance. Electron donating substituents increase electron density on benzene ring. Hereby they increase the stability of diazonium salts. Electron withdrawing substituents decrease electron density on benzene ring. Hereby they decrease the stability of diazonium salts. −COCH3 group is electron withdrawing and hence, diazonium salts from (D) is lessstable than that from (B). Although - O-COCH 3 iselectron donatingsubstituent, but it is present in metaposition. Hence, it will not have significant effect on stability. Therefore the increasing order of diazotisation is (A)<(D)<(C)<(B)