Final answer:
The pool operator should shock the pool with chlorine to break down irritating chloramines which are causing red eyes and a strong chlorine odor.
Step-by-step explanation:
In the scenario where swimmers are experiencing red eyes and there is a chlorine odor present in the pool, despite acceptable levels of pH, hardness, alkalinity, and chlorine, it is likely that the irritants are due to chloramines, which are formed when chlorine reacts with organic materials such as oils, sweat, and urine. The appropriate action for the pool operator in this case would be to shock the pool with chlorine (option A), which will break down the chloramines and sanitize the water effectively. This process, known as breakpoint chlorination, requires adding a large amount of chlorine in order to raise the chlorine level to a point where these combined chlorine molecules are broken apart. Adding calcium chloride (option B) would increase hardness but not address the chloramines. Adding soda ash (option C) would unnecessarily raise the pH, and adding sodium bicarbonate (option D) would increase the alkalinity, neither of which are necessary given the current water balance.