pKa=−logKa, here Ka acid dissociation constant. More the Ka value, stronger is the acidic nature of the respective acid.
Hence, the lower pKa value denotes a more powerful acid. Phenols are more acidic than alcohols. Hence, ethanol has more pKa value. Nitro group is the electron withdrawing group, hence it increases the acidic nature of phenols. Among the given phenols para nitrophenol is stronger acid. The pKa values are matched below.
Ethanol →15.9
Phenol→10
m−Nitrophenol →8.3
p−Nitrophenol→7.1
Hence, the answer is option D.
