Step 1: Understanding the Question:
Amines are classified as primary (\( 1^{\circ} \)), secondary (\( 2^{\circ} \)), or tertiary (\( 3^{\circ} \)) based on the number of carbon atoms directly attached to the nitrogen atom.
Step 3: Detailed Explanation:
(A) Cyclohexanamine: \( \text{C}_6\text{H}_{11}\text{NH}_2 \). Nitrogen is attached to 1 carbon. It is a Primary amine.
(B) Ethane-1,2-diamine: \( \text{NH}_2\text{CH}_2\text{CH}_2\text{NH}_2 \). Both nitrogens are attached to 1 carbon. They are Primary amine groups.
(C) N-Phenylbenzeneamine: Also known as diphenylamine. Nitrogen is attached to 2 phenyl rings. It is a Secondary amine.
(D) N-Ethyl-N-Methylpropan-2-amine: Nitrogen is attached to an ethyl group, a methyl group, and a propan-2-yl group. Total 3 carbon attachments. It is a Tertiary amine.
Step 4: Final Answer:
The tertiary amine is N-Ethyl-N-Methylpropan-2-amine.