Arrange the following labelled hydrogens in decreasing order of acidity:

Acidic strength$\propto$ Stability of conjugate base General Order of acidic strength:
$\mathrm{R}-\mathrm{COOH}>\mathrm{Ph}-\mathrm{OH}>\mathrm{R}-\mathrm{C}\equiv \mathrm{CH}$
'$\mathrm{C}$' is more acidic due to$-\mathrm{M}$ effect of$-{\mathrm{NO}}_2$ .

Hence , order of acidic strength is: b>c>d>a