A few drops of concentrated HNO3 is added to an unknown solution and boiled for a while. If this produces a brown solution, the cation presents are likely to be

Explanation: When a few drops of concentrated nitric acid (\(\ce{HNO3}\)) are added and the solution is boiled:

• \(\ce{Fe^{2+}}\) ions are oxidized by \(\ce{HNO3}\) to \(\ce{Fe^{3+}}\).
• \(\ce{Fe^{3+}}\) ions in solution give a characteristic yellow to brown coloration, due to the formation of ferric nitrate or hydrolyzed ferric species.

Check other options:

• Pb\(^{2+}\): does not give a brown solution under this treatment.
• Cu\(^{2+}\): gives a blue solution, not brown.
• Fe\(^{3+}\): already brown/yellow, but the key test is that unknown Fe\(^{2+}\) solution turns brown after oxidation by nitric acid.