$\mathrm{pKa}=-\mathrm{logKa}$

$\mathrm{Ka}\uparrow \mathrm{pKa}\downarrow$ & vice versa

Acidic character $\mathrm{\alpha }\mathrm{ }\mathrm{Ka}\mathrm{ }\mathrm{\alpha }\frac{1}{\mathrm{pKa}}$

Acidic character α stability of anion (Conjugate base)

Conjugate base are


Stability of conjugate base $\mathrm{\alpha }-\mathrm{M} \mathrm{or} -\mathrm{I}$ 
$\mathrm{\alpha }\frac{1}{+\mathrm{M}} \mathrm{or}\frac{1}{+\mathrm{I}}$
$-\mathrm{M}$ is more dominating than $-\mathrm{I}$

$\left(B\right)$   $-{\mathrm{NO}}_2$ at para so exerts $-\mathrm{M}$ or $-\mathrm{I}$  &  $-\mathrm{M}$ > $-\mathrm{I}$

So it is highly stable anion in

$\left(C\right)$ $-{\mathrm{NO}}_2$ at meta exerts only  $-\mathrm{I}$ effect

$\left(A\right)$ No $e^{-}$ withdrawing or electron releasing group.

$\left(D\right)$ $-{\mathrm{OCH}}_3$ has $+\mathrm{M}\mathrm{ }\&\mathrm{ }-\mathrm{I}$ but $+\mathrm{M}$ dominates

So acidic strength will be

$B>C>A>D$

Therefore $pKa$ values will be in the order

$B<C<A<D$