The formation of phenol from benzene diazonium chloride is a -------.A. Hydrolysis reactionB. Pyrolysis reactionC. Photosynthesis reactionD. Combustion reaction

The formation of phenol from benzene diazonium chloride involves the following reaction:

\[
\text{C}_6\text{H}_5\text{N}_2\text{Cl} + \text{H}_2\text{O} \rightarrow \text{C}_6\text{H}_5\text{OH} + \text{HCl} + \text{N}_2
\]

In this reaction, benzene diazonium chloride reacts with water (hydrolysis) to produce phenol (C₆H₅OH), hydrochloric acid (HCl), and nitrogen gas (N₂).

### Explanation of the Options:

1. **A. Hydrolysis reaction**: This is the correct answer. Hydrolysis is a reaction where water is used to break down a compound. In this case, the benzene diazonium chloride is hydrolyzed to form phenol.

2. **B. Pyrolysis reaction**: This refers to the thermal decomposition of materials at elevated temperatures in an inert atmosphere. It does not apply here since the reaction involves water.

3. **C. Photosynthesis reaction**: This is the process by which green plants convert light energy into chemical energy. This is not relevant to the formation of phenol.

4. **D. Combustion reaction**: This involves the burning of a substance in the presence of oxygen, typically producing heat and light. This is not applicable to the formation of phenol from benzene diazonium chloride.

### Conclusion:
The correct answer is **A. Hydrolysis reaction**.