Answer;
CaO + 3C → CaC2 + CO - Balanced
Na + H2O → 2NaOH + H2 - Not balanced
4Fe + O2 → 2Fe2O3 -Not balanced
2Mg + O2 → 2MgO - Balanced
Step-by-step explanation;
Balancing chemical equations is a process of trial and error.
The chemical equation needs to be balanced so that it follows the law of conservation of mass. A balanced chemical equation occurs when the number of the different atoms of elements in the reactants side is equal to that of the products side.