Answer
D.
Work Step by Step
Activation energy is a small amount of energy added to reactants to initiate a reaction, like a small push given to start something rolling downhill, which it will then do spontaneously. A. refers to the constant motion of molecules, not to the specifically needed activation energy, and B. talks about the downhill progress, at least for one kind of reaction, not to activation energy. Finally, C. refers to the total downhill drop, not to the small amount needed to start the reaction.
