Arrange in decreasing order of acid strength: (I) 4-nitrobenzoic acid, (II) benzoic acid, (III) 4-methoxybenzoic acid.

The -M and -I effects of $-\mathrm{NO}{}_2$ stabilise the carboxylate and raise acidity. The +M effect of $-\mathrm{OCH}{}_3$ at the para position destabilises the carboxylate by pushing electron density towards it, lowering acidity. The pKa values are about 3.4, 4.2 and 4.5, so the order is I > II > III.