Which of the following constituents contributes to water hardness?

Water hardness primarily results from the presence of specific dissolved minerals in water, with calcium being one of the key contributors. Calcium ions are naturally occurring in various geological formations and are soluble in water, leading to an increase in water hardness as they accumulate. Hard water often contains both calcium and magnesium ions, but calcium is specifically recognized for its significant role in defining water hardness levels.

In practical terms, water hardness affects many aspects of water use, including the formation of scale in plumbing and appliances, as well as its interaction with soaps and detergents. While other ions like iron and copper can be present in water and may influence its quality or cause staining, they do not contribute to water hardness in the same way that calcium does. Thus, calcium is the appropriate answer when identifying constituents that contribute to water hardness.