Among the following phenols, the most acidic is

Deprotonation of $\mathrm{p}-\mathrm{nitrophenol}$ gives p−nitrophenoxide ion. The negative charge on O atom is stabilised as strongly electron withdrawing nitro group is present in para position.

This effect is some what less in m−nitrophenol as  strongly electron withdrawing nitro group is present in meta position.

In case of p−aminophenol, deprotonation gives p−aminophenoxide ion. 

The negative charge on O atom is destabilised as electron releasing amino group is present in para position.

p−nitrophenol is most acidic.

Electron-withdrawal group at para position makes phenol more acidic.