As we know, benzenediazonium salt forms brilliant coloured dye with $\beta$-naphthol, the compound under consideration must be $p$-toludine (c) as it is a primary aromatic amine. Primary aromatic amine, on treatment with $\mathrm{NaNO}_2$ in dil. $\mathrm{HCl}$ forms the corresponding diazonium chloride salt.