In the final chemical equation nacl and o2 are the products that are formed through the reaction between na2o and cl2. before you can add these intermediate chemical equations you need to alter them by

In the above problem, the chemical equation would be
na₂o(aq) + cl₂(g) → nacl(s) + o₂(g) where aq means that the substance is in aqueous form, g is in gaseous form and s is in solid form. for this reaction to proceed, it needs addition of heat by boiling the solution. because without it, the solution would proceed so slowly that it might seem that there is no reaction.

The mass has escaped into a gas which you can not see so the law stays true.

Answer:   d.  carbon dioxide.