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.)