Which reactant acts as an acid by contributing a H+ ion to create H3O+ in the chemical equation?

Phenol (C₆H₅OH) reactant acts as acid in the chemical equation.

Explanation:

Phenol (C₆H₅OH) is the chemical equation.

The reaction is C₆H₅OH(aq) + H₂O(l) → C₆H₅O⁻(aq) + H₃O⁺(aq).

The reactant phenol (C₆H₅OH) has neutral charge and as product of reaction has negative charge (1-), So in this chemical reaction phenol gives one proton (H⁺) to the water and water became hydronium ion (H₃O⁺).

C6H5OH is the acid because, on the product side, it becomes C6H5O-. this means it lost a hydrogen, and this is what acids do they give hydrogen away.  

Phenol is acting as an acid as it's conjugate base is more stabilized due to resonance. Whereas if  ion is donated by water molecule then it will give a hydroxide ion which is not a strong base.

Therefore, here phenol is acting as an acid as it is donating an ion.