What causes temporary hardness in water?

Temporary hardness in water is primarily caused by the presence of dissolved calcium hydrogencarbonate. This compound can easily dissolve in water and is responsible for the formation of hard water that can be removed through boiling. When water containing calcium hydrogencarbonate is heated, it decomposes to form calcium carbonate, which precipitates out of the solution, thereby reducing the hardness.

This process explains why the term "temporary hardness" is used; it can be alleviated by simply boiling the water and thus removing the calcium as solid calcium carbonate. The other options, such as dissolved magnesium sulfate, calcium carbonate, and sodium chloride, do not contribute to temporary hardness in the same way. Sodium chloride, for instance, contributes to salinity rather than hardness, while magnesium sulfate and calcium carbonate would either create permanent hardness or not affect hardness in the context of this question.