Kjeldahl method cannot be used for :

Options

A.
B.
C.
D.

Explanation

To determine which compound the Kjeldahl method cannot be used for, we need to understand the principles of the Kjeldahl method and the types of nitrogen-containing compounds it can analyze.

### Step-by-Step Solution:

1. **Understanding the Kjeldahl Method**:
- The Kjeldahl method is a widely used method for determining the nitrogen content in organic compounds. It involves the digestion of the sample with concentrated sulfuric acid, converting nitrogen into ammonium sulfate.

2. **Identifying the Types of Nitrogen**:
- The method is effective for compounds that contain nitrogen in amine (–NH2) or amide (–CONH2) forms, as these can be converted to ammonium sulfate.
- However, the Kjeldahl method cannot be used for certain types of nitrogen, particularly those that are part of functional groups that do not yield ammonium sulfate upon digestion.

3. **Nitrogen Groups that Inhibit the Kjeldahl Method**:
- The method cannot be applied to compounds containing:
- Nitro groups (–NO2)
- Azo groups (–N=N–)
- Nitrogen in aromatic rings (e.g., pyridine)

4. **Analyzing the Given Options**:
- If we are given options such as aniline, nitrobenzene, cyanide (CN), and others, we need to identify which of these contains the problematic nitrogen groups.
- **Example Analysis**:
- **Aniline (C6H5NH2)**: Contains an amine group, Kjeldahl method can be used.
- **Nitrobenzene (C6H5NO2)**: Contains a nitro group, Kjeldahl method cannot be used.
- **Cyanide (CN)**: Can yield ammonium sulfate, Kjeldahl method can be used.
- **Other Compounds**: Evaluate similarly.

5. **Conclusion**:
- Based on the analysis, the compound that contains a nitro group (like nitrobenzene) is the one for which the Kjeldahl method cannot be applied.

### Final Answer:
The Kjeldahl method cannot be used for **Nitrobenzene**.