What element's presence in water is measured as part of water hardness?

The measurement of water hardness primarily focuses on the concentration of calcium and magnesium ions present in the water. Calcium is a crucial factor because it contributes significantly to the scaling and deposits that can form in pipes and appliances. When water is described as "hard," it typically means it has a high concentration of these minerals, particularly calcium. This property affects not only the taste of the water but also its suitability for various uses, such as in domestic and industrial applications.

Fluoride is associated with dental health, sodium is a common component in water treatment processes and occurs in various salts, and chlorine is typically added for disinfection purposes but does not influence water hardness. Thus, calcium is the correct answer as it is the primary element that defines water hardness levels.