Explanation
Electron withdrawing group increases the acidity of benzoic acid, o-isomer will have higher acidity than -p position have more pronounce electron-withdrawing effect than $$-NO_2$$ group at -m position.
Hence the order of acidity is
$$o- NO_2C_6H_4COOH \ > \ p-NO_2C_6H_4COOH \ > m-NO_2C_6H_4COOH \ > \ Ph-COOH$$
Hence, option D is correct.
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