Identify the medium required for formation of p-Hydroxyazobenzene from benzene diazonium chloride and phenol.

The electrophilic aromatic substitution between benzene diazonium chloride and phenol requires specific pH conditions.

The diazonium cation $C_6H_5N_2^+$ acts as an electrophile, while phenol must be in its deprotonated phenoxide form $C_6H_5O^-$ to serve as an effective nucleophile.

A mild alkaline medium (typically pH $9$–$10$) facilitates this deprotonation, enhancing nucleophilicity and enabling efficient coupling at the para position.

Acidic conditions protonate the diazonium group or suppress phenol dissociation, while neutral or non-aqueous solvents lack the necessary basicity.

The correct condition is therefore a mild alkaline medium.