IIT JEE Advanced 2011 Paper 1

The acidity of the acids can be measured in terms of its pKa value, greater is the pKa value, weaker is the acid. The decreasing order of pKa values is
p-nitrophenol (pKa 7.15) > p-hydroxybenzoic acid (pKa 4.58)
> p-toluic acid (pKa 4.37)
> o-hydroxybenzoic acid (pKa 2.98)
The pKa value for o-hydroxybenzoic acid is the least due to ortho effect. Here both the groups are ortho to each other so due to steric repulsions it is the strongest acid among these.)