A tertiary amine is one where the nitrogen atom is bonded to three carbon atoms. In the given options:
- Option (A), CH$_3$–N–CH$_3$, is a tertiary amine because the nitrogen is bonded to two methyl groups.
- Option (B) is a primary amine (NH$_2$ group), option (C) is a secondary amine (one nitrogen-carbon bond), and option (D) is also a secondary amine.
Step 1: Identify the nitrogen bonded to three carbon atoms to determine the tertiary amine.