The behaviour of amines toward nitrous acid depends sharply on their class. A primary aliphatic amine reacts with cold nitrous acid to form a highly unstable diazonium salt that immediately decomposes, releasing nitrogen gas with brisk effervescence and forming alcohols and other products. Ethylamine, being primary, therefore shows this rapid evolution of nitrogen. A secondary amine such as N-methylethanamine instead gives an oily yellow N-nitrosamine and does not liberate nitrogen gas. A tertiary amine such as N,N-dimethylethanamine has no N–H and simply forms a soluble salt with the acid, again with no nitrogen evolution. Hence only the primary amine effervesces. The option naming all three is wrong because the reaction is diagnostic precisely because the three classes differ. This contrast is exactly how NCERT uses nitrous acid to distinguish amine classes, paralleling the Hinsberg classification and the differing products of each class. As a plausibility check, the instability of aliphatic diazonium salts even near 273 K, in contrast to the relatively stable aromatic ones, explains why the nitrogen escapes immediately rather than forming a usable salt, confirming the observed effervescence is unique to the primary aliphatic amine.