To decrease the pH of a base in a solution at equilibrium, you would want to decrease the concentration of hydroxide ions (OH⁻), as pH is inversely related to the concentration of hydrogen ions (H⁺) and directly related to the concentration of hydroxide ions in a basic solution.
Among the options provided, the change that would accomplish this is:
decreasing the concentration of OH⁻ ions
This would lead to an increase in the H⁺ concentration, thereby decreasing the pH of the solution.