I was just confused about what this was asking.
This is a topic from kinetics .Here we need to know difference between the catalyst and intermediate.
When you add all reactions, a catalyst and intermediate are not the part of final equation. They cancelled out on adding all reactions.
Now a catalyst is added as reactant first and formed as a product in another reaction.
An intermediate is formed first as product and then reused as the reactant.
YO2 is the reactant in first two steps and product in last two steps so YO2 is the catalyst.
YO is formed in first two steps and used up in last two steps. So YO is the intermediate.
ohh! That makes sense, thank you!