Which of the following factors contributes to water hardness the least?

Water hardness is primarily a measurement of the concentration of divalent and trivalent metal ions in water, specifically calcium and magnesium ions. These ions are the main contributors to water hardness, as they combine with carbonate and sulfate to form hardness compounds.

Sodium, in contrast, is not a contributor to water hardness. Instead, it is usually associated with soft water or treated water systems where the hardness ions have been replaced by sodium ions, often through ion exchange processes. As such, sodium does not increase water hardness and instead is more related to water salinity.

Iron can have an impact on aesthetic qualities of water and may be present in sources of hardness, but it does not significantly contribute to the classification of water as hard or soft, like calcium and magnesium. While some iron may be present in hard water sources, its presence generally does not affect the hardness measurement in the same way that calcium and magnesium do.

Thus, sodium contributes the least to water hardness, making it the correct answer in this context.