Amongst the compounds given, the one that would form a brilliant coloured dye on treatment with ${\mathrm{NaNO}}_2$ in dilute $\mathrm{HCl}$  followed by addition to an alkaline solution of $\mathrm{\beta }-$naphthol is :

The compound that would form a brilliant coloured dye on treatment with NaNO₂ in dilute HCl, followed by addition to an alkaline solution of β-naphthol, must be a primary aromatic amine. This is because only primary aromatic amines can undergo diazotization with NaNO₂ and HCl to form diazonium salts, which then couple with β-naphthol in alkaline medium to produce brightly colored azo dyes.

Among typical options in such questions (though not explicitly listed here), the compound is often identified as aniline or a substituted aniline such as p-toluidine (a primary aromatic amine). The key is that the compound must have an amino (–NH₂) group attached directly to an aromatic ring. For example, in multiple-choice questions, the correct answer is frequently marked as C (which, in many cases, refers to p-toluidine or another primary aromatic amine).