When an acid is added to water, it dissociates and releases hydrogen ions (H⁺). The presence of these hydrogen ions leads to a decrease in the concentration of hydroxide ions (OH⁻) because the product of the concentrations of hydrogen ions and hydroxide ions in water at equilibrium is constant (Kw = [H⁺][OH⁻] = 1.0 x 10⁻¹⁴ at 25°C).
Therefore, the correct answer is:
C. Decrease