To get the net ionic equations, the ions in the equation have to expressed into their ionic form
Mg2+ + 2NO3- + 2Na+ + CO32- Β -> Β MgCO3(s) + 2Na+ + 2NO3-
Cancel out the spectator ions (present in both sides of the reaction)
The result is
c) Mg2+(aq) + CO32β(aq) β MgCO3(s)