Which of the following acids has lowest `pK_(a)` value?
A. `underset(NO_(2))underset(|)CH_(2)-overset(O)overset(||)C-OH`
B. `underset(NO_(2))underset(|)CH_(2)-CH_(2)-overset(O)overset(||)C-OH`
C. `underset(Cl)underset(|)CH_(2)-overset(O)overset(||)C-OH`
D. `CH_(3)-underset(Cl)underset(|)overset(Cl)overset(|)C-overset(O)overset(||)C-OH`