In phenols, the presence of electron releasing groups decrease the acidity, whereas presence of electron withdrawing groups increase the acidity, compared to phenol.

Among the meta and para-nitrophenols, the latter is more acidic as the presence of –NO₂ group at para position stabilises the phenoxide ion to a greater extent than when it is present at meta position. Thus, correct order of acidity is :

(iv) > (iii) > (i) > (ii)