Which of the following phenols are stronger acids than phenol?

The nitro groups withdraw electrons (-I at the meta position, -I and -M at the para position) and stabilise the phenoxide ion. Chlorine at the para position is net electron withdrawing, because -I outweighs +M. The $-\mathrm{NH}{}_2$ group at the para position is a strong +M donor and makes the phenol a weaker acid (pKa 10.3 against 9.99).