b
In compound \((a)\), \(NO _{2}\) possesses the \(- M\) effect at para position due to which it is highly acidic than other three compounds. The compound \((b)\) has intermolecular hydrogen bonding and it is less acidic than \((a)\). In compound \((d)\), there is absence of an electron donating group, due to which it is more acidic than compound \((c)\), which has electron donating methyl group. Therefore, the correct order of acidic nature is \(a > b > d > c\)