What is the equilibrium expression for the auto-ionization of water, and what does the ion product constant (K_w) represent?

• A. K_w = [H₃O⁺][OH^-], representing the sum of the concentrations of hydronium and hydroxide ions.
• B. K_w = [H₂O], representing the concentration of water molecules.
• C. K_w = [H⁺][OH^-], representing the product of the concentrations of hydronium and hydroxide ions.
• D. K_w = [H⁺][H₂O], representing the product of the concentrations of hydrogen ions and water molecules.

Answer: (C)