Which of the following is a method to add acid to pool water?

Hydrochloric acid, also known as muriatic acid, is a commonly used method to lower pH levels in pool water. Maintaining proper pH balance is crucial for water clarity, swimmer comfort, and the effectiveness of chlorine sanitization. When the pool water's pH is too high, hydrochloric acid can effectively bring the pH back down to the ideal range (typically between 7.2 and 7.8), thus improving the overall quality of the water.

Other substances listed serve different purposes: calcium hydroxide is primarily used to raise pH and increase alkalinity; sodium bicarbonate is used to raise total alkalinity without significantly altering pH; and calcium chloride is added to increase calcium hardness in the water. Understanding the specific functions of these chemicals is essential for effective pool maintenance, ensuring that the right substances are used at the right times.