Which of the following is the most acidic?

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(D) The acidity of phenols is determined by the stability of the phenoxide ion formed after the loss of a proton. Electron-withdrawing groups $(EWG)$

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$p$-Nitrophenol

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(D) The acidity of phenols is determined by the stability of the phenoxide ion formed after the loss of a proton. Electron-withdrawing groups $(EWG)$ stabilize the phenoxide ion and increase acidity,while electron-donating groups $(EDG)$ destabilize it and decrease acidity.$1$. Phenol: Reference compound.$2$. $p$-Cresol: The $-CH_3$ group is an electron-donating group ($+I$ and hyperconjugation),which destabilizes the phenoxide ion,making it less acidic than phenol.$3$. $m$-Nitrophenol: The $-NO_2$ group is a strong electron-withdrawing group ($-I$ and $-M$). In the meta position,only the $-I$ effect operates,which increases acidity.$4$. $p$-Nitrophenol: The $-NO_2$ group at the para position exerts both $-I$ and $-M$ effects. The $-M$ effect is very strong in stabilizing the negative charge on the phenoxide ion through resonance. Since the $-M$ effect is stronger than the $-I$ effect,$p$-nitrophenol is the most acidic among the given options.

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