The most acidic compound among the following is:

The acidity of phenols is determined by the stability of the phenoxide ion formed after the loss of a proton.

1. Electron Withdrawing Groups (EWG): Substituents like the nitro group ($-NO_2$) exert electron-withdrawing inductive ($-I$) and resonance ($-R$) effects . These effects disperse the negative charge on the oxygen of the phenoxide ion, stabilising it and thus increasing the acidity of the parent phenol.
2. Electron Donating Groups (EDG): Alkyl groups like methyl ($-CH_3$) exert an electron-releasing inductive effect ($+I$) , which intensifies the negative charge on the phenoxide ion, destabilising it and decreasing acidity.
3. Comparison:

• o-Cresol: Contains a methyl group ($+I$ effect), making it less acidic than phenol.
• Phenol: Standard reference.
• p-Nitrophenol: Contains one $-NO_2$ group, making it more acidic than phenol.
• 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.