The strongest acid from the following is:

Step 1: Compare substituent effects on phenol acidity

Acidity of phenol increases when the phenoxide anion (conjugate base) is stabilized. EWG stabilize phenoxide; EDG destabilize it.

Step 2: Analyse each compound

• (a) 3-Nitrophenol: $\ce{-NO2}$ is a strong EWG. At meta position, it stabilizes phenoxide via inductive effect (-I). Although resonance effect (+M) of $\ce{NO2}$ cannot reach phenoxide O from meta position, the strong -I effect still makes this significantly more acidic than phenol. pKa ≈ 8.3 (most acidic in this set).

• (b) 4-Chlorophenol: $\ce{-Cl}$ is weakly EWG by inductive effect (-I) but weakly EDG by resonance (+M) at para. Net effect: slightly more acidic than phenol but less so than nitrophenol. pKa ≈ 9.4.

• (c) 3-Methylphenol (m-cresol): $\ce{-CH3}$ is an EDG (+I effect) → destabilizes phenoxide → less acidic than phenol. pKa ≈ 10.1.

• (d) Phenol: No substituent, pKa ≈ 10.0 (reference).

Acidity order: (a) > (b) > (d) > (c)

Strongest acid = 3-nitrophenol ✓

Key Points to Remember:

• $\ce{-NO2}$ (both -I and -M effects): most powerful at increasing acidity; even at meta (only -I) it increases acidity significantly.
• $\ce{-CH3}$ (EDG by +I): decreases acidity compared to phenol.
• $\ce{-Cl}$ at para: net weak EWG effect → slightly more acidic than phenol.