To decrease the pH of a base in a solution at equilibrium, the most relevant action would be:
Decreasing the concentration of OH– ions.
This is because pH is a measure of the hydrogen ion concentration in a solution, and since pH and pOH are related (pH + pOH = 14), decreasing the concentration of hydroxide ions (OH–) will increase the hydrogen ion concentration (H+), thereby decreasing the pH.
The other options either do not impact the pH directly or would tend to increase the pH in the case of increasing the strength of the base or solute concentration.