Please see attached question. Not clear what the right answer is. It says both b and d. I thought by changing path the catalyst lowers Ea and that should be more right than just catalyst forming intermediate. Pl advise

It can not b because catalyst is not formed first and then used up.
so b is incorrect because catalyst is added first and then regenerated.
a is also not correct as if it is one of the step then it is regenerated in overall reaction.
d is the answer because a catalyst lowers the transition state and is the part of the reaction as reactant as well as the product.
c is also out because it lowers the activation energy and do not raise it.