Correct Answer - B
Given, `NaX+MgCl_(2)overset(Delta)(rarr)`White ppt.
`NaHCO_(3)+MgCl_(2)rarrunderset("Soluble")(Mg(HCO_(3))_(2))+NaCl`
`Mg(HCO_(3))_(2)overset(Delta)(rarr)underset("White ppt".)(MgCO_(3))+H_(2)O+Co_(2)`
Hence, anion of the sodium salt is `HCO_(3)^(Θ)`.