Organic Chemistry – Some Basic Principles Part 1

In aromatic acids presence of electron withdrawing substituent e.g. −NO2 disperses the negative charge of the anion and stablises it and hence increases the acidity of the parent benzoic acid.
Further O-isomer will have higher acidity than corresponding m and p isomers. Since nitro group at p-position have more pronounced electron withdrawing than −NO2 group at m position hence the correct order is the one given above.