The correct option is A HCO−3
Sodium bicarbonate reacts with MgCl2 to give magnesium bicarbonate, Mg(HCO3)2 which on heating produces MgCO3 which is white in color.
Reactions:
MgCl2+2NaHCO3→Mg(HCO3)2+2NaCl
Mg(HCO3)2Δ−→MgCO3White precipitate+H2O+CO2
Hence, the sodium salt is sodium bicarbonate and the anion of sodium salt is HCO−3.