Acidity and pKa values are inversely related. Therefore, the increasing order of acidity and decreasing order of pKa will be same. Phenols are less acidic than carboxylic acids. Electron donating groups (like OCH3 ) decrease the acidity and electron withdrawing groups (like NO2 ) increase the acidity. Hence, the increasing order of pKa value is A<D<B<C.
