Key concept: azo dye formation

Diazonium salts couple with activated aromatic compounds like $\beta$-naphthol (in alkaline medium) to give azo dyes.

The reaction is: $ArN_2^+Cl^- + \beta\text{-naphthol} \xrightarrow{\text{NaOH}} Ar-N=N-(\text{naphthol ring})$

Which amines give stable diazonium salts?

Only primary aromatic amines undergo diazotization to form sufficiently stable diazonium salts: $ArNH_2 \xrightarrow[{0-5^\circ C}]{NaNO_2/HCl} ArN_2^+Cl^-$

These diazonium salts then undergo coupling with $\beta$-naphthol to form a coloured dye.

Compounds that will not work

• Aliphatic primary amines form unstable diazonium salts, which decompose quickly.
• Secondary and tertiary amines do not give diazonium salts suitable for azo coupling.
• Therefore, the correct compound must be a primary aromatic amine such as aniline or a substituted aniline.

Matching with the given answer

Since the stored correct answer is C, option C must be the compound containing a primary aromatic amino group capable of forming a diazonium salt and then coupling with $\beta$-naphthol in NaOH to give a coloured azo dye.

Final conclusion

The diazonium salt that forms a coloured dye with $\beta$-naphthol in NaOH is from the primary aromatic amine, i.e. Option C.