To balance the given unbalanced chemical equation:
2NaHCO3 → Na + CO2 + CO2
The products currently do not match the reactants in terms of the number of each atom.
The main process here involves decomposing sodium bicarbonate into its products, which is commonly carbon dioxide and water (H2O).
Therefore, adding H2O to the products can help in balancing the equation.
So, the correct answer is H2O.