Consider the following reactions:Ca(HCO3)2 + Na2CO3  → →  B + NaHCO3Mg(HCO3)2 + Na2CO3  → →  A + NaHCO3D + Na2CO3  → → B + NaClIdentify B, A and D and select the correct statement.B and A are water solubleD is responsible for temporary hardness of waterCompound B can be produced by heating calcium bicarbonateCompound D shows efflorescence

The reactions are double displacement reactions.

For the first reaction: Ca(HCO3)2 + Na2CO3 → CaCO3 + 2NaHCO3 Here, B is CaCO3 (Calcium Carbonate)

For the second reaction: Mg(HCO3)2 + Na2CO3 → MgCO3 + 2NaHCO3 Here, A is MgCO3 (Magnesium Carbonate)

For the third reaction, we know that B is CaCO3 and it is formed with NaCl. So, D must be CaCl2 (Calcium Chloride)

So, B is Calcium Carbonate, A is Magnesium Carbonate and D is Calcium Chloride.

Now, let's analyze the statements:

1. B and A are water soluble - This is incorrect. Both Calcium Carbonate and Magnesium Carbonate are not soluble in water.
2. D is responsible for temporary hardness of water - This is correct. Calcium Chloride is one of the compounds that cause temporary hardness of water.
3. Compound B can be produced by heating calcium bicarbonate - This is correct. When Calcium Bicarbonate is heated, it decomposes to form Calcium Carbonate, Water and Carbon Dioxide.
4. Compound D shows efflorescence - This is correct. Calcium Chloride is a salt that shows efflorescence (the process of losing water of crystallization to the atmosphere on exposure to air).

So, the correct statements are 2, 3 and 4.