In this compound the activating group is OH.So it will activate the ring and makes the compound more reactive towards electrophile.Here the electrophile is NO2 group.It will attack at the ortho and para position.
The acidic character of the compound is denoted by the liberation of H+ ion.So H+ will be liberated easily when NO2 is at para position,because it will make the compound more stable as compared to othere when H+ is liberated.When NO2 is at ortho position H od hydroxide and O of nitro group will form intramolecular hydrogen bond and thus makes the liberation of H+ ion difficult
Thus the most acidic compound is paranitro phenol
