Which of the following molecules has the highest acidity?

Solution

1. Acidity is determined by the stability of the conjugate base, the phenoxide ion.
2. Electron-withdrawing groups increase acidity by stabilizing the negative charge on the oxygen atom.
3. The nitro group at the para position exerts a strong $-I$ and $-M$ effect, significantly stabilizing the phenoxide.
4. The methoxy and methyl groups are electron-donating, which destabilize the conjugate base.
5. Hence the answer is (B).