Which of the following aryl bromides cannot be converted directly into the corresponding arylmagnesium bromide by treatment with $\mathrm{Mg}$ in dry ether?

Any substituent with an acidic $\mathrm{O}-\mathrm{H}$ or $\mathrm{N}-\mathrm{H}$ hydrogen protonates the Grignard carbanion as it forms. The carboxylic acid and the primary aromatic amine have such hydrogens. The methyl and methoxy substituents have none, so these bromides give Grignard reagents normally.