Which of the following is the least 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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$o$-Cresol

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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 $(EWG)$ like $-NO_2$ and $-Cl$ stabilize the phenoxide ion through $-I$ and $-M$ effects,thereby increasing acidity. Electron-donating groups $(EDG)$ like $-CH_3$ (present in $o$-cresol) destabilize the phenoxide ion through $+I$ and hyperconjugation effects,thereby decreasing acidity. Comparing the options: $1$. $p$-Nitrophenol: Strongest acid due to the powerful $-M$ effect of $-NO_2$. $2$. $p$-Chlorophenol: Acidic due to the $-I$ effect of $-Cl$. $3$. Phenol: Baseline acidity. $4$. $o$-Cresol: Least acidic due to the electron-donating $+I$ effect of the methyl group. Therefore,$o$-cresol is the least acidic.

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