Compound responsible for temporary hardness of water is

Correct option is A

Solution
The hardness of water: The hardness of water is that characteristic that prevents the formation of sufficient leather or foam.
· It occurs due to the presence of carbonates, bicarbonates, sulphates, chlorides of  calcium and magnesium(Ca and Mg).
· Hardness of water is due to the presence of dissolved minerals of Ca2+, Mg2+, Al3+, Iron and other heavy elements. Specifically due to Calcium Sulphate, Calcium Bicarbonate, Calcium chloride, Magnesium Sulphate, Magnesium Bicarbonate and Magnesium chloride.​
Key Points
Hardness is of following two types-
Temporary hardness (Carbonate hardness):
· Temporary hardness is caused by the presence of dissolved  bicarbonates of Calcium (Ca(HCO3)2 ), Magnesium (Mg(HCO3)2  ), Iron and other heavy elements.
· It  can be eliminated by mere boiling.
Ca(HCO3)2 →  CaCO3↓ + H2O + CO2 ↑
Mg(HCO3)2 → MgCO3 + H2O + CO2 ↑
MgCO3 → Mg(OH)2↓ + CO2 ↑
Permanent Hardness (non-carbonate hardness):
· Permanent Hardness is due to the presence of dissolved  chlorides and sulphates of Calcium, Magnesium, Iron and other heavy elements.
· It cannot be eliminated by boiling.
Hence, we can conclude that  Mg(HCO3)2 is responsible for  temporary hardness of water.