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Weakest Acid

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ANSWER

B.

Understanding how to solve these type of problems where a benzene has various substituents that alter its pH depends on understanding electron-withdrawing substituents (fluorine and chlorine) and electron-donating substituents (hydroxyl and methyl, with the former being more powerful). The electron-donating   substituents destabilize the nitrogen’s lone pair of electrons, thus increasing its reactivity and making it more basic (less acidic).