Answer: the last option, there must be as equal number of atoms of each element on both sides of the equation.
Explanation:
During a chemical reaction, the elements (kind of atoms) are not transformed into different elements. Each element has to keep its identity. Sodium atoms will remain being sodium atoms, and the same is true for the 118 elements listed in the periodic table (all the elements known).
The chemical change (chemical reaction) consists in the rupture of the chemical bonds in the reactant compounds and the formation of new bonds to constitute one or more new compound.
Atoms are not created nor destroyed, in a chemical reaction, they just bond in different ways which is how different compounds are formed.
Since, atoms are not created nor destroyed there must be the same number of atoms of each element at the begining (reactant side) than at the end (product side).
That is the basis for balancing equations: you must make sure that equal number of atoms of each element on both sides of the equation.
