The given answer is 4; bit I couldn't find any place stating so. Meanwhile, Google says :-
that only small amount of Mg(OH)2 is formed. According to which 2 can be right. Anyway, since Ca and Mg bicarbonates are responsible for temorary hardness of water we also have to consider answer can be A also. Also, googling "what happens if we heat a sample of water which has temporary hardness" gives
What do you think?