o-and p-nitrophenols are stronger acids than m-nitrophenol. As a result, nitrophenols (a) and (c) are stronger acids than (b) and (d). In (c), the NO2 group is flanked by two CH3 groups which push the NO2 group out of the plane of the benzene ring. As a result of this steric hindrance the electron withdrawing resonance effect of the nitro group will be reduced (also called steric inhibition resonance) and hence the acidic character of the nitrophenol will decreases. Therefore, nitrophenol (a) is the strongest acid.