Arrange in decreasing order of acid strength: (I) 2,6-dimethylbenzoic acid, (II) benzoic acid, (III) 4-methylbenzoic acid.

The two ortho methyl groups twist the carboxyl group out of the plane of the ring. This steric inhibition of resonance removes the conjugation of $-\mathrm{COOH}$ with the ring, which would otherwise stabilise the undissociated acid, and the acid becomes much stronger (ortho effect). The para methyl group is electron releasing and makes the acid slightly weaker than benzoic acid. The order is I > II > III.