During reaction mechanism...
A. catalyst does not change the reaction mechanism.
B. catalyst changes the reaction mechanism.
C. catalyst always makes a reaction mechanim more complicated.
Catalysts lower the activation energy and provide alternative pathways to increase the rate of reaction. So A can't be right. But both B and C can be. I suspect B is the answer your prof wants but the mechanism COULD be more complicated. I suppose the secret word in C is "always". Perhaps it isn't always more complicated.