In which of the following, functional group isomerism is not possible ?

Options

A. Alcohols
B. Aldehydes
C. Alkyl halides
D. Cyanides

Explanation

To determine in which of the following options functional group isomerism is not possible, we need to analyze each option based on the definition of functional group isomerism. Functional group isomerism occurs when two or more compounds have the same molecular formula but different functional groups.

### Step-by-Step Solution:

1. **Understanding Functional Group Isomerism**:
- Functional group isomerism occurs when compounds have the same molecular formula but different functional groups. For example, alcohols and ethers can have the same molecular formula (C3H8O) but different functional groups.

2. **Analyzing Option A: Alcohols**:
- Alcohols (e.g., propanol, CH3CH2OH) can show functional group isomerism with ethers (e.g., dimethyl ether, CH3OCH3). Therefore, alcohols can exhibit functional group isomerism.

3. **Analyzing Option B: Aldehydes**:
- Aldehydes (e.g., formaldehyde, CH2O) can show functional group isomerism with ketones (e.g., acetone, C3H6O). Thus, aldehydes can also exhibit functional group isomerism.

4. **Analyzing Option C: Alkyl Halides**:
- Alkyl halides (e.g., chloroethane, C2H5Cl) consist of halogen atoms (like Cl, Br, F) attached to carbon. There are no different functional groups that can be formed from alkyl halides, so they do not exhibit functional group isomerism. Therefore, alkyl halides cannot form functional group isomers.

5. **Analyzing Option D: Cyanides**:
- Cyanides (e.g., acetonitrile, CH3CN) can show functional group isomerism with isocyanides (e.g., methyl isocyanide, CH3N≡C). Thus, cyanides can also exhibit functional group isomerism.

### Conclusion:
Based on the analysis, the option in which functional group isomerism is not possible is **C: Alkyl Halides**.

### Final Answer:
**C: Alkyl Halides**