You have to cancel out your spectator ions to get the net ionic equation
Molecular equation : CoCO3 (s) + 2HBr (aq) --> CoBr2 (aq) + H2O (l) + CO2 (g)
Net ionic: CoCO
3 + 2H
+ --> Co
2+ + H
2O + CO
2Note how the bromide ion is missing from the net ionic equation as it was a spectator ion.
So, I would choose D