$\text {The correct decreasing order of } \mathrm{p} K_a \text { is }$
$\text {The correct decreasing order of } \mathrm{p} K_a \text { is }$
II $>$ IV $>$ I $>$ III
IV $>$ II $>$ III $>$ I
III $>$ II $>$ IV $>$ I
IV $>$ I $>$ II $>$ III
Solution
Weaker acids have higher $\mathrm{p} K_a \cdot-\mathrm{OCH}_3$ at meta-position exerts only $-I$ effect, hence increase the acidity.
$-I$ effect order : $-\mathrm{NO}_2 > -\mathrm{OCH}_3 > -\mathrm{Cl}$.
$-\mathrm{CH}_3$ has $+I$ effect.
So order is (1).
