The carbylamine reaction, also called the isocyanide test, is a characteristic test for primary amines. When a primary amine is heated with chloroform and alcoholic potassium hydroxide, it forms a foul-smelling alkyl isocyanide. Mechanistically, KOH and chloroform generate dichlorocarbene, a highly electrophilic species, which is attacked by the lone pair of the primary amine; subsequent elimination of two molecules of HCl produces the isocyanide, in which carbon is bonded to nitrogen as R–N≡C. So ethylamine gives ethyl isocyanide. The nitrile option is wrong because nitriles have the connectivity R–C≡N, not the nitrogen-bonded carbon of an isocyanide, and they do not form under these conditions. The amide group does not arise here since no carbonyl source for an amide is present. The nitro group requires an oxidising nitration environment, which is absent. Only primary amines give this reaction, so it is also a diagnostic distinction from secondary and tertiary amines, exactly as stated in NCERT. As a consistency check, the diagnostic foul odour and the requirement of an N–H-bearing primary amine both confirm an isocyanide rather than any oxygen- or nitrile-containing product.