Which of the following is the most acidic?

Nitro groups at the ortho and para positions stabilise the phenoxide ion by both -M and -I effects. 2,4-Dinitrophenol has two such groups (pKa about 4.1), compared with 7.15 for 4-nitrophenol, 8.35 for 3-nitrophenol (only -I) and about 10.2 for 4-methylphenol.