Answer:
4. The number of atoms of each element
Explanation:
<em><u>Why is 1 wrong?</u></em>
The number of substances (i.e. number of mols of substances) do not have to be the same. For instance, 2 mol of water reacts with 1 mol of oxygen to form 2 mol of water. Obviously the numer of substances is not conserved. Thus, it isn't a requirement.
<em><u>Why is 2 wrong?</u></em>
The number of chemical symbols do not mean anything. They do not tell us anything about the quantity of matter, they only tell us about the elements that are involved in the reaction.
<em><u>Why is 3 wrong?</u></em>
Once again, the number of chemical formulas are meaningless.
<u><em>Why is 4 correct?</em></u>
The number of atoms are always conserved as atoms cannot be broken or created.
<em><u>Possible confusion...</u></em>
The number of atoms are always conserved but the number of molecules may not always be conserved.
Why? As 2 atoms may be a molecule on the reactant side and 4 atoms may become a molecule on the product size, causing the number of molecules to reduce by half, while the number of atoms remain constant.
Hope that makes sense!
