The strongest acid from the following is

Solution

Chemistry ma’am · 4 steps

Judge each phenol by its phenoxide ion: the group that pulls the negative charge away best wins.

1. Look at the anion. A phenol is acidic because losing H⁺ gives a phenoxide ion, ArO⁻. Anything that spreads out or pulls away that negative charge makes the ion more stable, so the phenol becomes a stronger acid.
2. Meta means inductive. All three groups here sit at the meta position. From meta, resonance cannot place charge on the ring carbon that carries the substituent, so only the inductive effect matters. A -I group raises acidity and a +I group lowers it.

   Option	Group at meta	Effect on phenoxide	pKₐ
   (a) m-cresol	-CH₃, +I	pushes in charge, destabilises	10.1
   (c) phenol	none	reference	10.0
   (d) m-chlorophenol	-Cl, -I	pulls charge, stabilises	9.1
   (b) m-nitrophenol	-NO₂, strong -I	pulls charge most strongly	8.4

3. Why nitro beats chloro. In -NO₂ the nitrogen carries a formal positive charge and is bonded to two electronegative oxygens. That makes its -I pull much stronger than that of a single Cl atom.
4. So. The order of acidity is m-nitrophenol > m-chlorophenol > phenol > m-cresol. The strongest acid is (b), m-nitrophenol.

The idea to keep. For substituted phenols, stabilise the phenoxide ion. At meta only -I or +I counts. At ortho and para, -R groups such as -NO₂ add a resonance boost on top.

Answer from the NTA final answer key. Solution by PrepWiser.