Final answer:
Adding a strong base to a solution will have the greatest effect on increasing the pH, while removing a strong base will have the greatest effect on decreasing the pH, because strong bases fully dissociate in water, affecting the concentration of OH− ions significantly.
Step-by-step explanation:
The question asks which action would have the greatest effect on the pH of a solution: adding a weak acid, adding a strong base, removing a weak acid, or removing a strong base. The pH of a solution is an indicator of its acidity or basicity, with lower pH values indicating higher acidity and higher pH values indicating higher basicity. Strong acids and bases have a more significant effect on pH because they completely dissociate in water, releasing a large number of H+ ions in the case of strong acids and OH− ions in the case of strong bases. Therefore, adding a strong base to a solution would typically have the greatest effect on increasing the pH (making it more basic), while removing a strong base would decrease the pH (making it more acidic).