The correct answer is: A. Hardness in water.
EDTA is a chelating agent, which means it can bind to metal ions. In the case of water hardness, EDTA binds to calcium and magnesium ions, which are the two main causes of water hardness. This binding prevents the calcium and magnesium ions from reacting with other substances, such as soap, which can cause problems like scale buildup and poor lather.
Turbidity is a measure of the cloudiness of water. It is caused by the presence of suspended particles in the water, such as dirt, clay, and algae. EDTA does not affect turbidity.
Dissolved oxygen is the amount of oxygen that is dissolved in water. It is important for aquatic life, as it is used in respiration. EDTA does not affect dissolved oxygen.
Residual chlorine is the amount of chlorine that is left in water after it has been treated with chlorine. It is used to kill bacteria and other microorganisms. EDTA does not affect residual chlorine.