Why is catalyst used in a chemical reaction?

One of them is right answer:

a)Because it increases the activation energy so that the reaction happens faster

b)Because it lowers the activation energy so that the reaction takes place more slowly

c)Because the reaction then always becomes exothermic and thus happens faster

d)Because it lowers the activation energy so that the reaction happens faster.

C(s) + H2O(g) + CO(g) + H2 (9) AH = 131 kJ