What does the ion product constant for water (K_w) represent?

• A. The difference between H⁺ and OH^- concentrations in water.
• B. The sum of H⁺ and OH^- concentrations in water.
• C. The ratio of H⁺ to OH^- concentrations in water.
• D. The product of H⁺ and OH^- concentrations in water.

Answer: (D)