Follow the nitrogen group as it changes at each arrow.
First arrow (Fe/HCl): This is a classic reduction. The pair Fe + HCl supplies hydrogen that knocks the oxygen off the nitro group and adds hydrogen, turning \(-NO_2\) into \(-NH_2\). Nitrobenzene therefore becomes aniline, \(C_6H_5NH_2\), which is (A).
Second arrow (\(NaNO_2\) + HCl, 273 K): Mixing sodium nitrite with hydrochloric acid makes nitrous acid on the spot. Nitrous acid attacks the amino group and rebuilds it as a diazonium group, \(-N_2^{+}\). The ice-cold temperature is essential, since the salt decomposes if it warms up. The product (B) is benzenediazonium chloride, \(C_6H_5N_2^{+}Cl^{-}\).
Third arrow (water/\(H^+\), heat): On boiling with water in acidic medium, the diazonium group is a very good leaving group; it departs as nitrogen gas and a hydroxyl group takes its place on the ring. That gives phenol, \(C_6H_5OH\), which is (C), with \(N_2\) bubbling off.
So the sequence marches nitrobenzene → aniline → benzenediazonium chloride → phenol.
\(\boxed{A:\ C_6H_5NH_2,\quad B:\ C_6H_5N_2^{+}Cl^{-},\quad C:\ C_6H_5OH}\)