**Benzene diazonium chloride**

Benzene diazonium chloride (C6H5N2Cl) is an organic compound that is commonly used in diazotization reactions. It is formed by the reaction of aniline (C6H5NH2) with nitrous acid (HNO2) in the presence of hydrochloric acid (HCl).

**Reaction with Phenol**

When benzene diazonium chloride reacts with phenol (C6H5OH) in a weakly basic medium, it undergoes a substitution reaction known as the Sandmeyer reaction. This reaction involves the replacement of the diazonium group (-N2Cl) with the phenolic group (-OH) to form a new compound.

**Formation of p-Hydroxyazobenzene**

The reaction between benzene diazonium chloride and phenol results in the formation of p-hydroxyazobenzene (C12H10N2O), which is the correct answer (option A).

The reaction proceeds as follows:

1. The weakly basic medium provides the necessary conditions for the reaction to occur. It helps in the deprotonation of phenol to form the phenoxide ion (C6H5O-).

2. The diazonium group of benzene diazonium chloride is highly reactive and undergoes nucleophilic substitution. The nitrogen atom of the diazonium group attacks the phenoxide ion, leading to the formation of a new carbon-nitrogen bond.

3. The chlorine atom attached to the nitrogen atom is replaced by the phenoxide group, resulting in the formation of p-hydroxyazobenzene.

The reaction can be represented by the following equation:

C6H5N2Cl + C6H5O- → C12H10N2O + Cl-

**Explanation of Other Options**

Option B: Benzenedoes not participate in the reaction. It remains unchanged.

Option C: Diphenyl ether is not formed in this reaction. It involves the formation of a carbon-oxygen bond between two phenyl groups, which is not observed in the given reaction.

Option D: Chlorobenzene is not formed in this reaction. The chlorine atom from the diazonium chloride is replaced by the phenoxide group, not by another chlorine atom.

Thus, the correct answer is option A: p-Hydroxyazobenzene.