Isn't the rate of Rxn is the rate of the slow step or do have to write the overall equation

Hi Mary. You're right, the rate of the reaction is indeed the rate law for the slowest step. Notice though, that this question isn't asking about what IS the rate law, but what COULD be the rate law. It means that there's something flawed about the wrong answers. You can't include intermediates in a rate law, because those aren't things you can increase/decrease to then affect the rate of the reaction. It has to be some sort of reactant that is added to the reaction. Therefore, the only molecules that can be included (but doesn't have to be all) is Cl2, Xe, and CO. All of the wrong answers have at least one intermediate as a part of the rate law. Hope that helps!