Azo-dye tests are used to detect the presence of primary aromatic amines. Let's break down the answer and understand why option 'D' is the correct choice.

Primary Aromatic Amines:
Primary aromatic amines are a type of amine compound where the amino group (-NH2) is directly attached to an aromatic ring. These compounds are typically derived from benzene or other aromatic compounds.

Azo-Dye Test:
The azo-dye test is a chemical test that is used to detect the presence of primary aromatic amines. Azo dyes are a class of synthetic dyes that contain azo groups (-N=N-) as a chromophore. When primary aromatic amines react with certain diazonium salts, azo dyes are formed.

Explanation of the Answer:
Now let's understand why the correct answer is option 'D' - "Only primary aromatic amines".

- All Amines: The statement 'all amines' is incorrect because the azo-dye test is specifically designed to detect primary aromatic amines, not all amines. Azo dyes are not formed when primary aliphatic amines or secondary/tertiary amines react with diazonium salts.

- Only Primary Amines: The statement 'only primary amines' is incorrect as well. The azo-dye test is not limited to primary amines. It specifically detects primary aromatic amines and not primary aliphatic amines.

- Only Primary Aliphatic Amines: The statement 'only primary aliphatic amines' is incorrect. As mentioned earlier, the azo-dye test is not specific to primary aliphatic amines. It is designed to detect primary aromatic amines.

- Only Primary Aromatic Amines: This statement is correct. The azo-dye test is given by primary aromatic amines. When primary aromatic amines react with diazonium salts, azo dyes are formed. This reaction is the basis of the azo-dye test.

Therefore, the correct answer to the question is option 'D' - "Only primary aromatic amines" because the azo-dye test specifically detects the presence of primary aromatic amines.