Why is activation energy required to start some reactions?

a.)to break the bonds of an endothermic reaction
b.)to overcome electron repulsion of an endothermic reaction
c.)to overcome electron repulsion of an exothermic reaction
d.)to provide sufficient energy to keep a reaction going

D.) to provide sufficient energy to keep a reaction going

Explanation:

Activation energy is the extra energy that must be supplied to reactants in order to cross the energy barrier and thus convert to products.

A catalyst is a substance which increases the rate of a reaction by taking the reaction through a different path which involves lower activation energy and thus more molecules can cross the energy barrier and convert to products.  It is required for both endothermic and exothermic reactions.

The catalyst itself does not take part in the chemical reaction and is regenerated as such at the end.