Consider the following compounds:
(i) p - methyl aniline (ii) N,N - dimethylaniline
(iii) N-ethyl aniline (iv) N-ethyl-N-methyl aniline
The compounds which do not form diazonium salt with ice - colds `NaNO_2` and HCl are:

Options

A. (i), (ii) and (iii)
B. (ii), (iii) and (iv)
C. (i), (iii) and (iv)
D. (ii) and (iv)

Explanation

To determine which of the given compounds do not form diazonium salts when treated with ice-cold NaNO2 and HCl, we need to analyze each compound based on its structure and classification as primary, secondary, or tertiary amines.

### Step-by-Step Solution:

1. **Identify the Compounds**:
- (i) p-Methyl aniline
- (ii) N,N-Dimethylaniline
- (iii) N-Ethyl aniline
- (iv) N-Ethyl-N-methyl aniline

2. **Classify Each Compound**:
- **p-Methyl aniline**: This is a primary amine (NH2 group attached to a benzene ring with a methyl group at the para position).
- **N,N-Dimethylaniline**: This is a tertiary amine (the nitrogen is bonded to three carbon atoms).
- **N-Ethyl aniline**: This is a secondary amine (the nitrogen is bonded to two carbon atoms).
- **N-Ethyl-N-methyl aniline**: This is also a tertiary amine (the nitrogen is bonded to two carbon atoms and one methyl group).

3. **Reactions with NaNO2 and HCl**:
- **Primary Amines**: React with NaNO2 and HCl to form diazonium salts. Therefore, p-methyl aniline will form a diazonium salt.
- **Secondary Amines**: React with NaNO2 and HCl to form nitroso compounds instead of diazonium salts. Therefore, N-ethyl aniline will not form a diazonium salt.
- **Tertiary Amines**: Do not react to form diazonium salts. Both N,N-dimethylaniline and N-ethyl-N-methyl aniline will not form diazonium salts.

4. **Conclusion**:
- The compounds that do not form diazonium salts are:
- N,N-Dimethylaniline (ii)
- N-Ethyl aniline (iii)
- N-Ethyl-N-methyl aniline (iv)

### Final Answer:
The compounds which do not form diazonium salt with ice-cold NaNO2 and HCl are:
- (ii) N,N-Dimethylaniline
- (iii) N-Ethyl aniline
- (iv) N-Ethyl-N-methyl aniline