Which option is used to lower the pH levels in pool water?

To effectively lower the pH levels in pool water, the use of an acid is the most appropriate choice. Acids, such as muriatic acid (hydrochloric acid) or sodium bisulfate, are commonly used for this purpose because they contribute hydrogen ions (H+) to the water. As more hydrogen ions are added, the concentration of hydrogen ions increases, resulting in a decrease in pH. Maintaining the proper pH is crucial for pool water balance, which affects chlorine efficiency, swimmer comfort, and overall water clarity.

Other substances mentioned serve different purposes; for example, sodium bicarbonate is actually used to raise alkalinity and pH levels rather than lower them. Chlorine is primarily used as a disinfectant and does not have a direct effect on pH levels. Algaecides are chemical agents used to control algae growth in pools and also do not impact the pH level directly. Hence, the use of acid is the most effective and recommended method for lowering pH levels in pool water.