The acidity of phenols depends on the stability of the phenoxide ion formed after the loss of a proton.

1. Resonance and Inductive Effects: Electron-withdrawing groups (EWG) like nitro ($-NO_2$) stabilize the phenoxide ion through resonance ($-R$ effect) and inductive ($-I$ effect) mechanisms, dispersing the negative charge and increasing acidity .
2. Number of Substituents: The more electron-withdrawing groups present, the greater the stabilization of the phenoxide ion.
3. Comparison:

• Phenol: No substituents.
• p-Nitrophenol: One $-NO_2$ group (stabilizes via $-I$ and $-R$).
• m-Nitrophenol: One $-NO_2$ group (stabilizes via $-I$ only, as resonance effect does not operate effectively from meta position).
• 2,4,6-Trinitrophenol (Picric acid): Contains three strong electron-withdrawing $-NO_2$ groups at ortho and para positions. The cumulative $-I$ and $-R$ effects provide maximum stabilisation to the phenoxide ion, making it the most acidic compound among the options (even more acidic than many carboxylic acids).