Which of the following is most acidic?

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(D) To determine the most acidic compound,we compare the acidity of carboxylic acids and phenols. Carboxylic acids are significantly more acidic than

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p-Nitrobenzoic acid

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(D) To determine the most acidic compound,we compare the acidity of carboxylic acids and phenols. Carboxylic acids are significantly more acidic than phenols due to the resonance stabilization of the carboxylate anion $(RCOO^-)$ compared to the phenoxide anion $(PhO^-)$.Among the given options:$A$ is Benzoic acid $(C_6H_5COOH)$.$B$ is $o$-Nitrophenol.$C$ is $p$-Nitrophenol.$D$ is $p$-Nitrobenzoic acid $(O_2N-C_6H_4-COOH)$.Comparing $A$ and $D$,both are carboxylic acids. The nitro group $(-NO_2)$ is a strong electron-withdrawing group $(EWG)$ due to both $-I$ and $-M$ effects. In $p$-Nitrobenzoic acid,the $-NO_2$ group at the para position exerts a strong electron-withdrawing effect,which stabilizes the carboxylate anion more effectively than in benzoic acid. Therefore,$p$-Nitrobenzoic acid is more acidic than benzoic acid.Since carboxylic acids are inherently more acidic than phenols,$p$-Nitrobenzoic acid is the most acidic among the choices.

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