Q.$C_6H_5NO_2 \xrightarrow{Sn/HCl} A \xrightarrow[273\ K]{NaNO_2/HCl} B \xrightarrow[283\ K]{H_2O} C$ 'C' is :

With loss of N2, the diazonium group is displaced by a nucleophile -- these are the routes to install substituents an aromatic ring otherwise resists. REPLACEMENT BY HYDROGEN: mild reducing agents (H3PO2/H2O, or ethanol with a CuCl catalyst) reduce the diazonium salt to benzene via a free-radical chain mechanism. REPLACEMENT BY Cl/Br/CN: the SANDMEYER REACTION (fresh diazonium salt + cuprous halide Cu2Cl2/Cu2Br2, or CuCN/KCN) gives the aryl halide or cyanobenzene + N2; the GATTERMANN REACTION achieves the same Cl/Br substitution with plain HCl/HBr + copper POWDER (no cuprous salt), generally at a lower yield than Sandmeyer. REPLACEMENT BY IODINE: aqueous diazonium salt + KI, warmed, gives iodobenzene directly -- no catalyst needed, unlike the Cl/Br/CN cases. REPLACEMENT BY FLUORINE (the BALZ-SCHIEMANN REACTION): diazonium chloride + HBF4 precipitates the stable diazonium tetrafluoroborate, which on heating decomposes to the aryl fluoride + BF3 + N2. REPLACEMENT BY HYDROXYL: diazonium salt added slowly to boiling water gives phenol + N2 + HCl. REPLACEMENT BY NITRO: diazonium fluoroborate + aqueous NaNO2/copper re …