Let's pass through each option.
Octet rule is a rule about the number of bonds an element can make, so it is related to structure of molecules, not reaction.
The Ideal Gas law has nothing to do with reaction, it works with conditions of ideal gases.
Both Law of Conservation of Mass and Law of Multiple Proportions have their part on chemical reactions.
Law of Multiple Proportions works with the proportions of the chemicals involved on the reactions and their stoichiometry, but not its balancing.
Law of Conservation of Mass says that no mass can be destroyied or created on a chemical reaction, and as a result, the reaction has to be balanced, so there are no elements appearing or desapearing.
So, the correct option is Law of Conservation of Mass