A chemical used to adjust pool alkalinity is:

Sodium bicarbonate, commonly known as baking soda, is frequently used in swimming pools to adjust alkalinity levels. The proper alkalinity levels help stabilize the pH of the water, making it less susceptible to fluctuations caused by environmental factors and swimmer usage. Maintaining appropriate alkaline levels is important for both the comfort of swimmers and the effectiveness of disinfectants like chlorine. When alkalinity is too low, pH levels can become unstable, leading to potential corrosion of pool equipment and irritation for swimmers. By using sodium bicarbonate, pool operators can effectively raise the total alkalinity, thus promoting better water balance and safety in the pool environment.

The other options do not serve the primary purpose of adjusting alkalinity as effectively. Chlorine is a disinfectant and primarily used for sanitizing the pool water. Calcium chloride is used for increasing calcium hardness rather than alkalinity, while copper sulfate is predominantly a algaecide and does not play a role in adjusting alkalinity.