Based on the data, what is the optimal amount of new fertilizer to maximize corn output?

8 or B

Explanation:

I think you are intended to answer 8, but would be very careful with this. The weather could have changed. There is a dramatic drop from 30 to 5 really requires careful study.

Fertilizer can be cumulative. That means that what you put on before could still be in the soil. Fertilizer then acts as a kind of poison. Some farmers grow crops that don't require fertilizer or that the use is minimal.  A crop that is an exception is something like Canola. It requires a great deal of fertilizer and it depletes the soil rather quickly. People grow it because it brings in a great deal of money per bushel and the soil returns many bushels per acre. But the soil depletes in nutrients rather quickly.