The sentence begins with a prepositional phrase (with corn_skyrocketing) modifying the main clause (grain futures_crops).
Grain futures brokers and their clients are very interested in weather that could affect the harvest of crops. Why are they interested? Because the reserves of corn, soybean, and wheat are quite low. How much low? Low to the extent that a poor harvest would make the prices extremely high. (Therefore, the brokers are interested in knowing whether there?s going to be a poor harvest)
Now, we understand that "a poor harvest would send price skyrocketing" is providing information about the extent of "low" levels of reserves. The correct and most concise way to express such relationship is using the idiom ?so X that Y? i.e. "so low that a poor harvest_" in option C.