Calcium carbonate can break down to form calcium oxide and carbon dioxide.
CaCO3 + CaO + CO2
The oxygen atoms on the product side are
not balanced because there are only two oxygen atoms in CO2
o not balanced because there is only one oxygen atom in Cao.
balanced because the total number of oxygen atoms is 3.
O balanced when a coefficient of 3 is placed in front of CO,.

The answer to your question is the third choice.

Explanation:

                        CaCO₃  ⇒   CaO  +  CO₂

Check if the reaction is balanced or unbalanced

                  Reactants       Elements         Products

                           1              Calcium                1

                           1               Carbon                 1

                           3              Oxygen                3

As the number of atoms in the reactant is the same that the number of atoms in the products we conclude that the reaction is balanced.

So the correct answer is balanced because the total number of oxygen atoms is 3.