To decrease the pH of a base in a solution at equilibrium, the most effective option would be:
decreasing the concentration of OH– ions
This is because pH is inversely related to the concentration of hydroxide ions (OH–) in a solution. Lowering the concentration of OH– ions will lead to an increase in hydrogen ion (H+) concentration, which in turn will decrease the pH.
The other options either do not directly lead to a decrease in pH or may even increase pH.