Acidity is directly proportional to conjugated base stability.


Stability order of conjugate bases is III > I > II > IV
Conjugate base in I is stabilised by - I > + M
Conjugate base in II is destabilised by + I and hyperconjugation.
Conjugate base in III is stabilised by strong withdrawing effect of −NO2 by - I & - M effect.
Conjugate base in IV is destabilised by + M effect of−OCH3
acidiccharacter∝−M,−I∝+M1,+HC1,+I1
Electron releasing group (i.e +I effect, +M) decreases the acidic strength
Electron withdrawing group (i.e -I effect,-M) increases acidic strength.

