**Core Concept**
Temporary hardness of water is a type of water hardness that can be removed by boiling. It is primarily caused by the presence of bicarbonate ions (HCO3-) in water, which are derived from the dissolution of carbonates and bicarbonates of calcium and magnesium.

**Why the Correct Answer is Right**
The bicarbonate ions (HCO3-) in water react with heat to form carbon dioxide gas (CO2) and water, thereby removing the temporary hardness. This reaction is as follows: Ca(HCO3)2 → CaCO3 (down) + CO2 (up) + H2O. The carbon dioxide gas is released into the atmosphere, and the calcium carbonate precipitates out of the solution, thereby removing the temporary hardness.

**Why Each Wrong Option is Incorrect**
**Option A:** Incorrect because bicarbonate ions are the primary cause of temporary hardness, not sodium ions.
**Option B:** Incorrect because sulfate ions can contribute to permanent hardness, not temporary hardness.
**Option C:** Incorrect because chloride ions do not contribute to water hardness.

**Clinical Pearl / High-Yield Fact**
Temporary hardness of water can be removed by boiling, but not permanent hardness, which requires demineralization or reverse osmosis.

**Correct Answer:** A. Bicarbonate ions.