Which of the following is the most acidic?

In 2,4-dinitrophenol both nitro groups are at positions conjugated with the phenoxide oxygen (ortho and para), so both exert -M and -I effects (pKa about 4.1). In 3,5-dinitrophenol both nitro groups are meta and exert only -I (pKa about 6.7). 4-Nitrophenol has pKa 7.15 and 2,4-dimethylphenol is weaker than phenol.