Aniline can be distinguished from ethanamine by

Nitrous acid test

The correct answer is Option (3) → Nitrous acid test

Aniline is an aromatic amine while ethanamine is an aliphatic amine, and both react differently with nitrous acid ($HNO_2$).

At room temperature:

Ethanamine (aliphatic amine) react with nitrous acid to form aliphatic diazonium salts which being unstable liberate nitrogen gas.

$ \text{R}-\text{NH}_2+\text{HNO}_2\overset{{\text{NaNO}_2+\text{HCl}}}{\longrightarrow}\left[\text{R}-\text{N}_2^{+}\text{Cl}^{-}\right]\overset{{\text{H}_2\text{O}}}{\longrightarrow}\text{ROH}+\text{N}_2\uparrow+\text{HCl} $

Aniline (aromatic amine) react with nitrous acid at low temperatures (273-278 K) to form diazonium salts

$ \text{C}_6\text{H}_5\text{NH}_2+\text{NaNO}_2+2\text{HCl}\overset{{\text{273}-\text{278 K}}}{\longrightarrow}\text{C}_6\text{H}_5-\text{N}_2^{+}\text{Cl}^{-}+\text{NaCl}+2\text{H}_2\text{O}$