Two step reaction

Which of the options is the catalyst for the following two-step reaction ?
Reaction : CL + 03 - CI0 + 0,
Reaction : CIO + 0 - Cl+ 02
A) Cl
B) 02
C) O
D) 03

The answer is A. May I know how?

In a two step reaction catalyst is something which is used up first and produced on later as we can clearly see here that Cl is on the reactant side in equation 1 while on the product side in reaction 2 so this is our catalyst.
You can also relate it in some other way:- Catalyst is something which doesn’t get used up in a reaction so, if you are adding a certain amount of catalyst in a particular reaction you gonna retrieve all of that once the reaction completes.
Hope it helps:)