Compound responsible for temporary hardness of water is

Hardness of water is caused by dissolved calcium and magnesium salts. Temporary hardness specifically comes from the bicarbonates (hydrogencarbonates) of calcium and magnesium — Ca(HCO₃)₂ and Mg(HCO₃)₂ — because these salts decompose into insoluble carbonates on boiling, which is why this hardness is removable and called 'temporary'. Permanent hardness, by contrast, is caused by the chlorides and sulfates of calcium and magnesium, which do not break down on boiling.

Among the given compounds, Mg(HCO₃)₂ is a bicarbonate salt of magnesium, so it is the compound responsible for temporary hardness of water.

This can be confirmed by the defining test for temporary hardness: boiling water containing Mg(HCO₃)₂ converts it to insoluble MgCO₃, which precipitates out — exactly the behaviour that distinguishes temporary hardness from permanent hardness.

• CaCl₂ is a chloride salt of calcium; chlorides do not decompose on boiling, so they cause permanent hardness.

• MgSO₄ is a sulfate salt of magnesium; sulfates likewise persist on boiling and also cause permanent hardness.

• CaCO₃ is only sparingly soluble in water; it is the insoluble precipitate formed when temporary hardness is removed by boiling, not a cause of hardness itself.