The correct option is
C 4>1>3>2More the stability of conjugate base, more is the acidic strength.
The conjugate base so formed after removal of
H+ is :
Between 1 and 4 :
In 1, the negative charge is delocalized and stabilized by only
−I effect of
−NO2. This is because
−M effect does not operate at meta-position.
In 4,
−M effect of
−NO2 increases the stability of conjugate base.
Since
−M effect is more dominating than
−I effect , 4 is more stable than 1
In 3, negative charge is delocalized with non-equivalent resonating structures, so it is less stable than 1 and 4.
In 2 , there is no delocalisation of negative charge, so it is the least stable, and least acidic.
So, the correct order is
4>1>3>2