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. $-\mathrm{COCH}_3$ group is electron withdrawing and hence, diazonium salts from (D) is less stable than that from (B). Although-O- $\mathrm{COCH}_3$ is electron donating substituent, but it is present in meta position. Hence, it will not have significant effect on stability. The increasing order of diazotisation is (A) $ < $ (D) $ < $ (B) $ < $ (C).