The answer is C. For C, it can react irreversibly with A. That means when adding C, the concentration of A will decrease. For reaction A-->B. if A decrease, then the reaction rate will decrease.
See where the liquid or whatever you're measuring meets with the numbers! And thats your answer!
I say C because why would you want a plant to resist something that helps it grow? A, B, and D make sense because those can improve the quality of the plant, but fertilizer can improve plant quality and help it grow. So there is no reason to resist it.
